libelf: check loops for running away

Ensure that libelf does not have any loops which can run away
indefinitely even if the input is bogus.  (Grepped for \bfor, \bwhile
and \bgoto in libelf and xc_dom_*loader*.c.)

Changes needed:
 * elf_note_next uses the note's unchecked alleged length, which might
   wrap round.  If it does, return ELF_MAX_PTRVAL (0xfff..fff) instead,
   which will be beyond the end of the section and so terminate the
   caller's loop.  Also check that the returned psuedopointer is sane.
 * In various loops over section and program headers, check that the
   calculated header pointer is still within the image, and quit the
   loop if it isn't.
 * Some fixed limits to avoid potentially O(image_size^2) loops:
    - maximum length of strings: 4K (longer ones ignored totally)
    - maximum total number of ELF notes: 65536 (any more are ignored)
 * Check that the total program contents (text, data) we copy or
   initialise doesn't exceed twice the output image area size.
 * Remove an entirely useless loop from elf_xen_parse (!)
 * Replace a nested search loop in in xc_dom_load_elf_symtab in
   xc_dom_elfloader.c by a precomputation of a bitmap of referenced
   symtabs.

We have not changed loops which might, in principle, iterate over the
whole image - even if they might do so one byte at a time with a
nontrivial access check function in the middle.

This is part of the fix to a security issue, XSA-55.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

v8: Fix the two loops in libelf-dominfo.c; the comment about
     PT_NOTE and SHT_NOTE wasn't true because the checks did
     "continue", not "break".
    Add a comment about elf_note_next's expectations of the caller's
     loop conditions (which most plausible callers will follow anyway).

v5: Fix regression due to wrong image size loop limit calculation.
    Check return value from xc_dom_malloc.

v4: Fix regression due to misplacement of test in elf_shdr_by_name
     (uninitialised variable).
    Introduce fixed limits.
    Avoid O(size^2) loops.
    Check returned psuedopointer from elf_note_next is correct.
    A few style fixes.

v3: Fix a whitespace error.

v2: BUGFIX: elf_shdr_by_name, elf_note_next: Reject new <= old, not just <.
    elf_shdr_by_name: Change order of checks to be a bit clearer.
    elf_load_bsdsyms: shdr loop check, improve chance of brokenness detection.
    Style fixes.

5 files changed, 135 insertions, 29 deletions

diff --git a/tools/libxc/xc_dom_elfloader.c b/tools/libxc/xc_dom_elfloader.c
index 75e469a451..3b835ee5d6 100644
--- a/tools/libxc/xc_dom_elfloader.c
+++ b/tools/libxc/xc_dom_elfloader.c
@@ -28,6 +28,7 @@
 
 #include "xg_private.h"
 #include "xc_dom.h"
+#include "xc_bitops.h"
 
 #define XEN_VER "xen-3.0"
 
@@ -118,6 +119,7 @@ static elf_errorstatus xc_dom_load_elf_symtab(struct xc_dom_image *dom,
     ELF_PTRVAL_CHAR hdr;
     size_t size;
     unsigned h, count, type, i, tables = 0;
+    unsigned long *strtab_referenced = NULL;
 
     if ( elf_swap(elf) )
     {
@@ -218,22 +220,35 @@ static elf_errorstatus xc_dom_load_elf_symtab(struct xc_dom_image *dom,
               symtab, maxaddr);
 
     count = elf_shdr_count(&syms);
+    /* elf_shdr_count guarantees that count is reasonable */
+
+    strtab_referenced = xc_dom_malloc(dom, bitmap_size(count));
+    if ( strtab_referenced == NULL )
+        return -1;
+    bitmap_clear(strtab_referenced, count);
+    /* Note the symtabs @h linked to by any strtab @i. */
+    for ( i = 0; i < count; i++ )
+    {
+        shdr2 = elf_shdr_by_index(&syms, i);
+        if ( elf_uval(&syms, shdr2, sh_type) == SHT_SYMTAB )
+        {
+            h = elf_uval(&syms, shdr2, sh_link);
+            if (h < count)
+                set_bit(h, strtab_referenced);
+        }
+    }
+
     for ( h = 0; h < count; h++ )
     {
         shdr = ELF_OBSOLETE_VOIDP_CAST elf_shdr_by_index(&syms, h);
+        if ( !elf_access_ok(elf, ELF_HANDLE_PTRVAL(shdr), 1) )
+            /* input has an insane section header count field */
+            break;
         type = elf_uval(&syms, shdr, sh_type);
         if ( type == SHT_STRTAB )
         {
-            /* Look for a strtab @i linked to symtab @h. */
-            for ( i = 0; i < count; i++ )
-            {
-                shdr2 = elf_shdr_by_index(&syms, i);
-                if ( (elf_uval(&syms, shdr2, sh_type) == SHT_SYMTAB) &&
-                     (elf_uval(&syms, shdr2, sh_link) == h) )
-                    break;
-            }
             /* Skip symtab @h if we found no corresponding strtab @i. */
-            if ( i == count )
+            if ( !test_bit(h, strtab_referenced) )
             {
                 if ( elf_64bit(&syms) )
                     elf_store_field(elf, shdr, e64.sh_offset, 0);
diff --git a/xen/common/libelf/libelf-dominfo.c b/xen/common/libelf/libelf-dominfo.c
index 0b0700260c..8ca2a33a8f 100644
--- a/xen/common/libelf/libelf-dominfo.c
+++ b/xen/common/libelf/libelf-dominfo.c
@@ -221,7 +221,8 @@ elf_errorstatus elf_xen_parse_note(struct elf_binary *elf,
 static unsigned elf_xen_parse_notes(struct elf_binary *elf,
                                struct elf_dom_parms *parms,
                                ELF_PTRVAL_CONST_VOID start,
-                               ELF_PTRVAL_CONST_VOID end)
+                               ELF_PTRVAL_CONST_VOID end,
+                               unsigned *total_note_count)
 {
     unsigned xen_elfnotes = 0;
     ELF_HANDLE_DECL(elf_note) note;
@@ -233,6 +234,12 @@ static unsigned elf_xen_parse_notes(struct elf_binary *elf,
           ELF_HANDLE_PTRVAL(note) < parms->elf_note_end;
           note = elf_note_next(elf, note) )
     {
+        if ( *total_note_count >= ELF_MAX_TOTAL_NOTE_COUNT )
+        {
+            elf_mark_broken(elf, "too many ELF notes");
+            break;
+        }
+        (*total_note_count)++;
         note_name = elf_note_name(elf, note);
         if ( note_name == NULL )
             continue;
@@ -473,6 +480,7 @@ elf_errorstatus elf_xen_parse(struct elf_binary *elf,
     ELF_HANDLE_DECL(elf_phdr) phdr;
     unsigned xen_elfnotes = 0;
     unsigned i, count, more_notes;
+    unsigned total_note_count = 0;
 
     elf_memset_unchecked(parms, 0, sizeof(*parms));
     parms->virt_base = UNSET_ADDR;
@@ -487,6 +495,9 @@ elf_errorstatus elf_xen_parse(struct elf_binary *elf,
     for ( i = 0; i < count; i++ )
     {
         phdr = elf_phdr_by_index(elf, i);
+        if ( !elf_access_ok(elf, ELF_HANDLE_PTRVAL(phdr), 1) )
+            /* input has an insane program header count field */
+            break;
         if ( elf_uval(elf, phdr, p_type) != PT_NOTE )
             continue;
 
@@ -499,7 +510,8 @@ elf_errorstatus elf_xen_parse(struct elf_binary *elf,
 
         more_notes = elf_xen_parse_notes(elf, parms,
                                  elf_segment_start(elf, phdr),
-                                 elf_segment_end(elf, phdr));
+                                 elf_segment_end(elf, phdr),
+                                 &total_note_count);
         if ( more_notes == ELF_NOTE_INVALID )
             return -1;
 
@@ -516,13 +528,17 @@ elf_errorstatus elf_xen_parse(struct elf_binary *elf,
         for ( i = 0; i < count; i++ )
         {
             shdr = elf_shdr_by_index(elf, i);
+            if ( !elf_access_ok(elf, ELF_HANDLE_PTRVAL(shdr), 1) )
+                /* input has an insane section header count field */
+                break;
 
             if ( elf_uval(elf, shdr, sh_type) != SHT_NOTE )
                 continue;
 
             more_notes = elf_xen_parse_notes(elf, parms,
                                      elf_section_start(elf, shdr),
-                                     elf_section_end(elf, shdr));
+                                     elf_section_end(elf, shdr),
+                                     &total_note_count);
 
             if ( more_notes == ELF_NOTE_INVALID )
                 return -1;
@@ -540,20 +556,15 @@ elf_errorstatus elf_xen_parse(struct elf_binary *elf,
      */
     if ( xen_elfnotes == 0 )
     {
-        count = elf_shdr_count(elf);
-        for ( i = 0; i < count; i++ )
+        shdr = elf_shdr_by_name(elf, "__xen_guest");
+        if ( ELF_HANDLE_VALID(shdr) )
         {
-            shdr = elf_shdr_by_name(elf, "__xen_guest");
-            if ( ELF_HANDLE_VALID(shdr) )
-            {
-                parms->guest_info = elf_section_start(elf, shdr);
-                parms->elf_note_start = ELF_INVALID_PTRVAL;
-                parms->elf_note_end   = ELF_INVALID_PTRVAL;
-                elf_msg(elf, "%s: __xen_guest: \"%s\"\n", __FUNCTION__,
-                        elf_strfmt(elf, parms->guest_info));
-                elf_xen_parse_guest_info(elf, parms);
-                break;
-            }
+            parms->guest_info = elf_section_start(elf, shdr);
+            parms->elf_note_start = ELF_INVALID_PTRVAL;
+            parms->elf_note_end   = ELF_INVALID_PTRVAL;
+            elf_msg(elf, "%s: __xen_guest: \"%s\"\n", __FUNCTION__,
+                    elf_strfmt(elf, parms->guest_info));
+            elf_xen_parse_guest_info(elf, parms);
         }
     }
 
diff --git a/xen/common/libelf/libelf-loader.c b/xen/common/libelf/libelf-loader.c
index 937c99b463..47957aa8f2 100644
--- a/xen/common/libelf/libelf-loader.c
+++ b/xen/common/libelf/libelf-loader.c
@@ -75,6 +75,9 @@ elf_errorstatus elf_init(struct elf_binary *elf, const char *image_input, size_t
     for ( i = 0; i < count; i++ )
     {
         shdr = elf_shdr_by_index(elf, i);
+        if ( !elf_access_ok(elf, ELF_HANDLE_PTRVAL(shdr), 1) )
+            /* input has an insane section header count field */
+            break;
         if ( elf_uval(elf, shdr, sh_type) != SHT_SYMTAB )
             continue;
         elf->sym_tab = shdr;
@@ -170,6 +173,9 @@ void elf_parse_bsdsyms(struct elf_binary *elf, uint64_t pstart)
     for ( i = 0; i < elf_shdr_count(elf); i++ )
     {
         shdr = elf_shdr_by_index(elf, i);
+        if ( !elf_access_ok(elf, ELF_HANDLE_PTRVAL(shdr), 1) )
+            /* input has an insane section header count field */
+            break;
         type = elf_uval(elf, shdr, sh_type);
         if ( (type == SHT_STRTAB) || (type == SHT_SYMTAB) )
             sz = elf_round_up(elf, sz + elf_uval(elf, shdr, sh_size));
@@ -224,6 +230,9 @@ do {                                            \
 
     for ( i = 0; i < elf_shdr_count(elf); i++ )
     {
+        elf_ptrval old_shdr_p;
+        elf_ptrval new_shdr_p;
+
         type = elf_uval(elf, shdr, sh_type);
         if ( (type == SHT_STRTAB) || (type == SHT_SYMTAB) )
         {
@@ -235,8 +244,16 @@ do {                                            \
              elf_hdr_elm(elf, shdr, sh_offset, maxva - symtab_addr);
              maxva = ELF_OBSOLETE_VOIDP_CAST elf_round_up(elf, (unsigned long)maxva + sz);
         }
-        shdr = ELF_MAKE_HANDLE(elf_shdr, ELF_HANDLE_PTRVAL(shdr) +
-                            (unsigned long)elf_uval(elf, elf->ehdr, e_shentsize));
+        old_shdr_p = ELF_HANDLE_PTRVAL(shdr);
+        new_shdr_p = old_shdr_p + elf_uval(elf, elf->ehdr, e_shentsize);
+        if ( new_shdr_p <= old_shdr_p ) /* wrapped or stuck */
+        {
+            elf_mark_broken(elf, "bad section header length");
+            break;
+        }
+        if ( !elf_access_ok(elf, new_shdr_p, 1) ) /* outside image */
+            break;
+        shdr = ELF_MAKE_HANDLE(elf_shdr, new_shdr_p);
     }
 
     /* Write down the actual sym size. */
@@ -256,6 +273,9 @@ void elf_parse_binary(struct elf_binary *elf)
     for ( i = 0; i < count; i++ )
     {
         phdr = elf_phdr_by_index(elf, i);
+        if ( !elf_access_ok(elf, ELF_HANDLE_PTRVAL(phdr), 1) )
+            /* input has an insane program header count field */
+            break;
         if ( !elf_phdr_is_loadable(elf, phdr) )
             continue;
         paddr = elf_uval(elf, phdr, p_paddr);
@@ -278,11 +298,20 @@ elf_errorstatus elf_load_binary(struct elf_binary *elf)
     ELF_HANDLE_DECL(elf_phdr) phdr;
     uint64_t i, count, paddr, offset, filesz, memsz;
     ELF_PTRVAL_VOID dest;
+    /*
+     * Let bizarre ELFs write the output image up to twice; this
+     * calculation is just to ensure our copying loop is no worse than
+     * O(domain_size).
+     */
+    uint64_t remain_allow_copy = (uint64_t)elf->dest_size * 2;
 
     count = elf_uval(elf, elf->ehdr, e_phnum);
     for ( i = 0; i < count; i++ )
     {
         phdr = elf_phdr_by_index(elf, i);
+        if ( !elf_access_ok(elf, ELF_HANDLE_PTRVAL(phdr), 1) )
+            /* input has an insane program header count field */
+            break;
         if ( !elf_phdr_is_loadable(elf, phdr) )
             continue;
         paddr = elf_uval(elf, phdr, p_paddr);
@@ -290,6 +319,20 @@ elf_errorstatus elf_load_binary(struct elf_binary *elf)
         filesz = elf_uval(elf, phdr, p_filesz);
         memsz = elf_uval(elf, phdr, p_memsz);
         dest = elf_get_ptr(elf, paddr);
+
+        /*
+         * We need to check that the input image doesn't have us copy
+         * the whole image zillions of times, as that could lead to
+         * O(n^2) time behaviour and possible DoS by a malicous ELF.
+         */
+        if ( remain_allow_copy < memsz )
+        {
+            elf_mark_broken(elf, "program segments total to more"
+                            " than the input image size");
+            break;
+        }
+        remain_allow_copy -= memsz;
+
         elf_msg(elf, "%s: phdr %" PRIu64 " at 0x%"ELF_PRPTRVAL" -> 0x%"ELF_PRPTRVAL"\n",
                 __func__, i, dest, (ELF_PTRVAL_VOID)(dest + filesz));
         if ( elf_load_image(elf, dest, ELF_IMAGE_BASE(elf) + offset, filesz, memsz) != 0 )
diff --git a/xen/common/libelf/libelf-tools.c b/xen/common/libelf/libelf-tools.c
index 6543f33615..ef13b0dd77 100644
--- a/xen/common/libelf/libelf-tools.c
+++ b/xen/common/libelf/libelf-tools.c
@@ -131,7 +131,16 @@ uint64_t elf_round_up(struct elf_binary *elf, uint64_t addr)
 
 unsigned elf_shdr_count(struct elf_binary *elf)
 {
-    return elf_uval(elf, elf->ehdr, e_shnum);
+    unsigned count = elf_uval(elf, elf->ehdr, e_shnum);
+    uint64_t max = elf->size / sizeof(Elf32_Shdr);
+    if (max > ~(unsigned)0)
+        max = ~(unsigned)0; /* Xen doesn't have limits.h :-/ */
+    if (count > max)
+    {
+        elf_mark_broken(elf, "far too many section headers");
+        count = max;
+    }
+    return count;
 }
 
 unsigned elf_phdr_count(struct elf_binary *elf)
@@ -149,6 +158,9 @@ ELF_HANDLE_DECL(elf_shdr) elf_shdr_by_name(struct elf_binary *elf, const char *n
     for ( i = 0; i < count; i++ )
     {
         shdr = elf_shdr_by_index(elf, i);
+        if ( !elf_access_ok(elf, ELF_HANDLE_PTRVAL(shdr), 1) )
+            /* input has an insane section header count field */
+            break;
         sname = elf_section_name(elf, shdr);
         if ( sname && !strcmp(sname, name) )
             return shdr;
@@ -204,6 +216,11 @@ const char *elf_strval(struct elf_binary *elf, elf_ptrval start)
         if ( !elf_access_unsigned(elf, start, length, 1) )
             /* ok */
             return ELF_UNSAFE_PTR(start);
+        if ( length >= ELF_MAX_STRING_LENGTH )
+        {
+            elf_mark_broken(elf, "excessively long string");
+            return NULL;
+        }
     }
 }
 
@@ -327,7 +344,14 @@ ELF_HANDLE_DECL(elf_note) elf_note_next(struct elf_binary *elf, ELF_HANDLE_DECL(
     unsigned namesz = (elf_uval(elf, note, namesz) + 3) & ~3;
     unsigned descsz = (elf_uval(elf, note, descsz) + 3) & ~3;
 
-    return ELF_MAKE_HANDLE(elf_note, ELF_HANDLE_PTRVAL(note) + elf_size(elf, note) + namesz + descsz);
+    elf_ptrval ptrval = ELF_HANDLE_PTRVAL(note)
+        + elf_size(elf, note) + namesz + descsz;
+
+    if ( ( ptrval <= ELF_HANDLE_PTRVAL(note) || /* wrapped or stuck */
+           !elf_access_ok(elf, ELF_HANDLE_PTRVAL(note), 1) ) )
+        ptrval = ELF_MAX_PTRVAL; /* terminate caller's loop */
+
+    return ELF_MAKE_HANDLE(elf_note, ptrval);
 }
 
 /* ------------------------------------------------------------------------ */
diff --git a/xen/include/xen/libelf.h b/xen/include/xen/libelf.h
index 87e6f40d10..63d056d014 100644
--- a/xen/include/xen/libelf.h
+++ b/xen/include/xen/libelf.h
@@ -51,6 +51,9 @@ typedef void elf_log_callback(struct elf_binary*, void *caller_data,
 
 #endif
 
+#define ELF_MAX_STRING_LENGTH 4096
+#define ELF_MAX_TOTAL_NOTE_COUNT 65536
+
 /* ------------------------------------------------------------------------ */
 
 /* Macros for accessing the input image and output area. */
@@ -353,6 +356,16 @@ ELF_PTRVAL_CONST_VOID elf_note_desc(struct elf_binary *elf, ELF_HANDLE_DECL(elf_
 uint64_t elf_note_numeric(struct elf_binary *elf, ELF_HANDLE_DECL(elf_note) note);
 uint64_t elf_note_numeric_array(struct elf_binary *, ELF_HANDLE_DECL(elf_note),
                                 unsigned int unitsz, unsigned int idx);
+
+/*
+ * If you use elf_note_next in a loop, you must put a nontrivial upper
+ * bound on the returned value as part of your loop condition.  In
+ * some cases elf_note_next will substitute ELF_PTRVAL_MAX as return
+ * value to indicate that the iteration isn't going well (for example,
+ * the putative "next" value would be earlier in memory).  In this
+ * case the caller's loop must terminate.  Checking against the
+ * end of the notes segment with a strict inequality is sufficient.
+ */
 ELF_HANDLE_DECL(elf_note) elf_note_next(struct elf_binary *elf, ELF_HANDLE_DECL(elf_note) note);
 
 /* (Only) checks that the image has the right magic number. */