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authorKevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>2018-12-21 15:18:39 +0000
committerKevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>2018-12-24 21:52:08 +0000
commit8f4841462cd392cc7dc0c1dba6c8c3a0d4b68910 (patch)
treeab6f4cfdca8d5f1afcb1e15cf23f2950fea15a34 /target/linux/generic/backport-4.19
parent87c5fd348d14de07735550c81d95fd20520055df (diff)
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kernel: MIPS: math-emu Write-protect delay slot emulation pages
Backport https://git.kernel.org/pub/scm/linux/kernel/git/mips/linux.git/commit/?id=adcc81f148d733b7e8e641300c5590a2cdc13bf3 "Mapping the delay slot emulation page as both writeable & executable presents a security risk, in that if an exploit can write to & jump into the page then it can be used as an easy way to execute arbitrary code. Prevent this by mapping the page read-only for userland, and using access_process_vm() with the FOLL_FORCE flag to write to it from mips_dsemul(). This will likely be less efficient due to copy_to_user_page() performing cache maintenance on a whole page, rather than a single line as in the previous use of flush_cache_sigtramp(). However this delay slot emulation code ought not to be running in any performance critical paths anyway so this isn't really a problem, and we can probably do better in copy_to_user_page() anyway in future. A major advantage of this approach is that the fix is small & simple to backport to stable kernels. Reported-by: Andy Lutomirski <luto@kernel.org> Signed-off-by: Paul Burton <paul.burton@mips.com> Fixes: 432c6bacbd0c ("MIPS: Use per-mm page to execute branch delay slot instructions")" Without patch: cat /proc/self/maps 00400000-0047a000 r-xp 00000000 1f:03 1823 /bin/busybox 00489000-0048a000 r-xp 00079000 1f:03 1823 /bin/busybox 0048a000-0048b000 rwxp 0007a000 1f:03 1823 /bin/busybox 77ec8000-77eed000 r-xp 00000000 1f:03 2296 /lib/libgcc_s.so.1 77eed000-77eee000 rwxp 00015000 1f:03 2296 /lib/libgcc_s.so.1 77eee000-77f81000 r-xp 00000000 1f:03 2470 /lib/libc.so 77f90000-77f92000 rwxp 00092000 1f:03 2470 /lib/libc.so 77f92000-77f94000 rwxp 00000000 00:00 0 7f946000-7f967000 rw-p 00000000 00:00 0 [stack] 7fefb000-7fefc000 rwxp 00000000 00:00 0 7ffac000-7ffad000 r--p 00000000 00:00 0 [vvar] 7ffad000-7ffae000 r-xp 00000000 00:00 0 [vdso] Patch applied: cat /proc/self/maps 00400000-0047a000 r-xp 00000000 1f:03 1825 /bin/busybox 00489000-0048a000 r-xp 00079000 1f:03 1825 /bin/busybox 0048a000-0048b000 rwxp 0007a000 1f:03 1825 /bin/busybox 77ed0000-77ef5000 r-xp 00000000 1f:03 2298 /lib/libgcc_s.so.1 77ef5000-77ef6000 rwxp 00015000 1f:03 2298 /lib/libgcc_s.so.1 77ef6000-77f89000 r-xp 00000000 1f:03 2474 /lib/libc.so 77f98000-77f9a000 rwxp 00092000 1f:03 2474 /lib/libc.so 77f9a000-77f9c000 rwxp 00000000 00:00 0 7fbed000-7fc0e000 rw-p 00000000 00:00 0 [stack] 7fefb000-7fefc000 r-xp 00000000 00:00 0 7fff6000-7fff7000 r--p 00000000 00:00 0 [vvar] 7fff7000-7fff8000 r-xp 00000000 00:00 0 [vdso] Note lack of write permission to 7fefb000-7fefc000 Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Diffstat (limited to 'target/linux/generic/backport-4.19')
-rw-r--r--target/linux/generic/backport-4.19/096-mips-math-emu-Write-protect-delay-slot-emulation-pages.patch119
1 files changed, 119 insertions, 0 deletions
diff --git a/target/linux/generic/backport-4.19/096-mips-math-emu-Write-protect-delay-slot-emulation-pages.patch b/target/linux/generic/backport-4.19/096-mips-math-emu-Write-protect-delay-slot-emulation-pages.patch
new file mode 100644
index 0000000000..f428285a64
--- /dev/null
+++ b/target/linux/generic/backport-4.19/096-mips-math-emu-Write-protect-delay-slot-emulation-pages.patch
@@ -0,0 +1,119 @@
+From adcc81f148d733b7e8e641300c5590a2cdc13bf3 Mon Sep 17 00:00:00 2001
+From: Paul Burton <paul.burton@mips.com>
+Date: Thu, 20 Dec 2018 17:45:43 +0000
+Subject: MIPS: math-emu: Write-protect delay slot emulation pages
+
+Mapping the delay slot emulation page as both writeable & executable
+presents a security risk, in that if an exploit can write to & jump into
+the page then it can be used as an easy way to execute arbitrary code.
+
+Prevent this by mapping the page read-only for userland, and using
+access_process_vm() with the FOLL_FORCE flag to write to it from
+mips_dsemul().
+
+This will likely be less efficient due to copy_to_user_page() performing
+cache maintenance on a whole page, rather than a single line as in the
+previous use of flush_cache_sigtramp(). However this delay slot
+emulation code ought not to be running in any performance critical paths
+anyway so this isn't really a problem, and we can probably do better in
+copy_to_user_page() anyway in future.
+
+A major advantage of this approach is that the fix is small & simple to
+backport to stable kernels.
+
+Reported-by: Andy Lutomirski <luto@kernel.org>
+Signed-off-by: Paul Burton <paul.burton@mips.com>
+Fixes: 432c6bacbd0c ("MIPS: Use per-mm page to execute branch delay slot instructions")
+Cc: stable@vger.kernel.org # v4.8+
+Cc: linux-mips@vger.kernel.org
+Cc: linux-kernel@vger.kernel.org
+Cc: Rich Felker <dalias@libc.org>
+Cc: David Daney <david.daney@cavium.com>
+---
+ arch/mips/kernel/vdso.c | 4 ++--
+ arch/mips/math-emu/dsemul.c | 38 ++++++++++++++++++++------------------
+ 2 files changed, 22 insertions(+), 20 deletions(-)
+
+--- a/arch/mips/kernel/vdso.c
++++ b/arch/mips/kernel/vdso.c
+@@ -126,8 +126,8 @@ int arch_setup_additional_pages(struct l
+
+ /* Map delay slot emulation page */
+ base = mmap_region(NULL, STACK_TOP, PAGE_SIZE,
+- VM_READ|VM_WRITE|VM_EXEC|
+- VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC,
++ VM_READ | VM_EXEC |
++ VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC,
+ 0, NULL);
+ if (IS_ERR_VALUE(base)) {
+ ret = base;
+--- a/arch/mips/math-emu/dsemul.c
++++ b/arch/mips/math-emu/dsemul.c
+@@ -214,8 +214,9 @@ int mips_dsemul(struct pt_regs *regs, mi
+ {
+ int isa16 = get_isa16_mode(regs->cp0_epc);
+ mips_instruction break_math;
+- struct emuframe __user *fr;
+- int err, fr_idx;
++ unsigned long fr_uaddr;
++ struct emuframe fr;
++ int fr_idx, ret;
+
+ /* NOP is easy */
+ if (ir == 0)
+@@ -250,27 +251,31 @@ int mips_dsemul(struct pt_regs *regs, mi
+ fr_idx = alloc_emuframe();
+ if (fr_idx == BD_EMUFRAME_NONE)
+ return SIGBUS;
+- fr = &dsemul_page()[fr_idx];
+
+ /* Retrieve the appropriately encoded break instruction */
+ break_math = BREAK_MATH(isa16);
+
+ /* Write the instructions to the frame */
+ if (isa16) {
+- err = __put_user(ir >> 16,
+- (u16 __user *)(&fr->emul));
+- err |= __put_user(ir & 0xffff,
+- (u16 __user *)((long)(&fr->emul) + 2));
+- err |= __put_user(break_math >> 16,
+- (u16 __user *)(&fr->badinst));
+- err |= __put_user(break_math & 0xffff,
+- (u16 __user *)((long)(&fr->badinst) + 2));
++ union mips_instruction _emul = {
++ .halfword = { ir >> 16, ir }
++ };
++ union mips_instruction _badinst = {
++ .halfword = { break_math >> 16, break_math }
++ };
++
++ fr.emul = _emul.word;
++ fr.badinst = _badinst.word;
+ } else {
+- err = __put_user(ir, &fr->emul);
+- err |= __put_user(break_math, &fr->badinst);
++ fr.emul = ir;
++ fr.badinst = break_math;
+ }
+
+- if (unlikely(err)) {
++ /* Write the frame to user memory */
++ fr_uaddr = (unsigned long)&dsemul_page()[fr_idx];
++ ret = access_process_vm(current, fr_uaddr, &fr, sizeof(fr),
++ FOLL_FORCE | FOLL_WRITE);
++ if (unlikely(ret != sizeof(fr))) {
+ MIPS_FPU_EMU_INC_STATS(errors);
+ free_emuframe(fr_idx, current->mm);
+ return SIGBUS;
+@@ -282,10 +287,7 @@ int mips_dsemul(struct pt_regs *regs, mi
+ atomic_set(&current->thread.bd_emu_frame, fr_idx);
+
+ /* Change user register context to execute the frame */
+- regs->cp0_epc = (unsigned long)&fr->emul | isa16;
+-
+- /* Ensure the icache observes our newly written frame */
+- flush_cache_sigtramp((unsigned long)&fr->emul);
++ regs->cp0_epc = fr_uaddr | isa16;
+
+ return 0;
+ }