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 <keesc...@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 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 <t...@linaro.org> > --- > 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 > >