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Subject[PATCH] MIPS: math-emu: Write-protect delay slot emulation pages
Date
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+
---
arch/mips/kernel/vdso.c | 4 ++--
arch/mips/math-emu/dsemul.c | 38 +++++++++++++++++++------------------
2 files changed, 22 insertions(+), 20 deletions(-)

diff --git a/arch/mips/kernel/vdso.c b/arch/mips/kernel/vdso.c
index 48a9c6b90e07..9df3ebdc7b0f 100644
--- a/arch/mips/kernel/vdso.c
+++ b/arch/mips/kernel/vdso.c
@@ -126,8 +126,8 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)

/* 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;
diff --git a/arch/mips/math-emu/dsemul.c b/arch/mips/math-emu/dsemul.c
index 5450f4d1c920..e2d46cb93ca9 100644
--- 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, mips_instruction ir,
{
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, mips_instruction ir,
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, mips_instruction ir,
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;
}
--
2.20.0
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