Messages in this thread | | | Date | Wed, 11 May 2016 08:30:23 -0700 | Subject | Re: [PATCH] arm64: kernel: Fix incorrect brk randomization | From | Kees Cook <> |
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On Wed, May 11, 2016 at 8:29 AM, Will Deacon <will.deacon@arm.com> wrote: > On Wed, May 11, 2016 at 08:27:14AM -0700, Kees Cook wrote: >> On Wed, May 11, 2016 at 2:44 AM, Jon Medhurst (Tixy) <tixy@linaro.org> wrote: >> > On Tue, 2016-05-10 at 10:55 -0700, Kees Cook wrote: >> >> This fixes two issues with the arm64 brk randomziation. First, the >> >> STACK_RND_MASK was being used incorrectly. The original code was: >> >> >> >> unsigned long range_end = base + (STACK_RND_MASK << PAGE_SHIFT) + 1; >> >> >> >> STACK_RND_MASK is 0x7ff (32-bit) or 0x3ffff (64-bit), with 4K pages where >> >> PAGE_SHIFT is 12: >> >> >> >> #define STACK_RND_MASK (test_thread_flag(TIF_32BIT) ? \ >> >> 0x7ff >> (PAGE_SHIFT - 12) : \ >> >> 0x3ffff >> (PAGE_SHIFT - 12)) >> >> >> >> This means the resulting offset from base would be 0x7ff0001 or 0x3ffff0001, >> >> which is wrong since it creates an unaligned end address. It was likely >> >> intended to be: >> >> >> >> unsigned long range_end = base + ((STACK_RND_MASK + 1) << PAGE_SHIFT) >> >> >> >> Which would result in offsets of 0x800000 (32-bit) and 0x40000000 (64-bit). >> >> >> >> However, even this corrected 32-bit compat offset (0x00800000) is much >> >> smaller than native ARM's brk randomization value (0x02000000): >> >> >> >> unsigned long arch_randomize_brk(struct mm_struct *mm) >> >> { >> >> unsigned long range_end = mm->brk + 0x02000000; >> >> return randomize_range(mm->brk, range_end, 0) ? : mm->brk; >> >> } >> >> >> >> So, instead of basing arm64's brk randomization on mistaken STACK_RND_MASK >> >> calculations, just use specific corrected values for compat (0x2000000) >> >> and native arm64 (0x40000000). >> >> >> >> Signed-off-by: Kees Cook <keescook@chromium.org> >> > >> > There seems to be a helper 'is_compat_task()' that does >> > 'test_thread_flag(TIF_32BIT)' so could perhaps be used instead, but >> > that's too nit-picky. This change makes things more consistent with >> >> Oh, good call. Yeah, none of the other .c code does direct tests for >> the TIF_32BIT flag, so I'll use the helper and send a v2. Thanks! > > I already applied it with that change and Tixy's reviewed-by. Thanks!
Oh! Perfect, thanks! :)
-Kees
-- Kees Cook Chrome OS & Brillo Security
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