libelf: Check pointer references in elf_is_elfbinary

elf_is_elfbinary didn't take a length parameter and could potentially
access out of range when provided with a very short image.

We only need to check the size is enough for the actual dereference in
elf_is_elfbinary; callers are just using it to check the magic number
and do their own checks (usually via the new elf_ptrval system) before
dereferencing other parts of the header.

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

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

v7: Add a comment about the limited function of elf_is_elfbinary.

v2: Style fix.
    Fix commit message subject.

2 files changed, 7 insertions, 4 deletions

diff --git a/xen/common/libelf/libelf-loader.c b/xen/common/libelf/libelf-loader.c
index 878552ef08..6c43c34619 100644
--- a/xen/common/libelf/libelf-loader.c
+++ b/xen/common/libelf/libelf-loader.c
@@ -29,7 +29,7 @@ int elf_init(struct elf_binary *elf, const char *image_input, size_t size)
     ELF_HANDLE_DECL(elf_shdr) shdr;
     uint64_t i, count, section, offset;
 
-    if ( !elf_is_elfbinary(image_input) )
+    if ( !elf_is_elfbinary(image_input, size) )
     {
         elf_err(elf, "%s: not an ELF binary\n", __FUNCTION__);
         return -1;
diff --git a/xen/common/libelf/libelf-tools.c b/xen/common/libelf/libelf-tools.c
index 08ab0279de..b613593786 100644
--- a/xen/common/libelf/libelf-tools.c
+++ b/xen/common/libelf/libelf-tools.c
@@ -332,11 +332,14 @@ ELF_HANDLE_DECL(elf_note) elf_note_next(struct elf_binary *elf, ELF_HANDLE_DECL(
 
 /* ------------------------------------------------------------------------ */
 
-int elf_is_elfbinary(const void *image)
+int elf_is_elfbinary(const void *image_start, size_t image_size)
 {
-    const Elf32_Ehdr *ehdr = image;
+    const Elf32_Ehdr *ehdr = image_start;
 
-    return IS_ELF(*ehdr); /* fixme unchecked */
+    if ( image_size < sizeof(*ehdr) )
+        return 0;
+
+    return IS_ELF(*ehdr);
 }
 
 int elf_phdr_is_loadable(struct elf_binary *elf, ELF_HANDLE_DECL(elf_phdr) phdr)