On Wed, May 11, 2016 at 2:44 AM, Jon Medhurst (Tixy) <[email protected]> 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 <[email protected]> > > 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! -Kees > arch/arm and other arches, and stops use of STACK_RND_MASK for things > that aren't stack related so seems good all round to me. > > Reviewed-by: Jon Medhurst <[email protected]> > >> --- >> arch/arm64/kernel/process.c | 15 ++++++++------- >> 1 file changed, 8 insertions(+), 7 deletions(-) >> >> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c >> index 80624829db61..0d0969bcd76d 100644 >> --- a/arch/arm64/kernel/process.c >> +++ b/arch/arm64/kernel/process.c >> @@ -382,13 +382,14 @@ unsigned long arch_align_stack(unsigned long sp) >> return sp & ~0xf; >> } >> >> -static unsigned long randomize_base(unsigned long base) >> -{ >> - unsigned long range_end = base + (STACK_RND_MASK << PAGE_SHIFT) + 1; >> - return randomize_range(base, range_end, 0) ? : base; >> -} >> - >> unsigned long arch_randomize_brk(struct mm_struct *mm) >> { >> - return randomize_base(mm->brk); >> + unsigned long range_end = mm->brk; >> + >> + if (test_thread_flag(TIF_32BIT)) >> + range_end += 0x02000000; >> + else >> + range_end += 0x40000000; >> + >> + return randomize_range(mm->brk, range_end, 0) ? : mm->brk; >> } >> -- >> 2.6.3 >> >> > > -- Kees Cook Chrome OS & Brillo Security
