| Message ID | 1449856338-30984-5-git-send-email-dcashman@android.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 12/11/15 09:52, Daniel Cashman wrote: > From: dcashman <dcashman@google.com> > > x86: arch_mmap_rnd() uses hard-coded values, 8 for 32-bit and 28 for > 64-bit, to generate the random offset for the mmap base address. > This value represents a compromise between increased ASLR > effectiveness and avoiding address-space fragmentation. Replace it > with a Kconfig option, which is sensibly bounded, so that platform > developers may choose where to place this compromise. Keep default > values as new minimums. > > Signed-off-by: Daniel Cashman <dcashman@android.com> OK, this is around the time when I make a lecture about the danger of expecting the compiler to make certain transformations: > diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c > index 844b06d..647fecf 100644 > --- a/arch/x86/mm/mmap.c > +++ b/arch/x86/mm/mmap.c > @@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void) > { > unsigned long rnd; > > - /* > - * 8 bits of randomness in 32bit mmaps, 20 address space bits > - * 28 bits of randomness in 64bit mmaps, 40 address space bits > - */ > if (mmap_is_ia32()) > - rnd = (unsigned long)get_random_int() % (1<<8); > +#ifdef CONFIG_COMPAT > + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits); > +#else > + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits); > +#endif > else > - rnd = (unsigned long)get_random_int() % (1<<28); > + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits); > > return rnd << PAGE_SHIFT; > } > Now, you and I know that both variants can be implemented with a simple AND, but I have a strong suspicion that once this is turned into a variable, this will in fact be changed from an AND to a divide. So I'd prefer to use the "get_random_int() & ((1UL << mmap_rnd_bits) - 1)" construct instead. -hpa
On 12/14/2015 10:58 AM, H. Peter Anvin wrote: > On 12/11/15 09:52, Daniel Cashman wrote: >> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c >> index 844b06d..647fecf 100644 >> --- a/arch/x86/mm/mmap.c >> +++ b/arch/x86/mm/mmap.c >> @@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void) >> { >> unsigned long rnd; >> >> - /* >> - * 8 bits of randomness in 32bit mmaps, 20 address space bits >> - * 28 bits of randomness in 64bit mmaps, 40 address space bits >> - */ >> if (mmap_is_ia32()) >> - rnd = (unsigned long)get_random_int() % (1<<8); >> +#ifdef CONFIG_COMPAT >> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits); >> +#else >> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits); >> +#endif >> else >> - rnd = (unsigned long)get_random_int() % (1<<28); >> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits); >> >> return rnd << PAGE_SHIFT; >> } >> > > Now, you and I know that both variants can be implemented with a simple > AND, but I have a strong suspicion that once this is turned into a > variable, this will in fact be changed from an AND to a divide. > > So I'd prefer to use the > "get_random_int() & ((1UL << mmap_rnd_bits) - 1)" construct instead. Good point. Will change in v7 across patch-set. Thank You, Dan
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index db3622f..bdc67d2 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -82,6 +82,8 @@ config X86 select HAVE_ARCH_KASAN if X86_64 && SPARSEMEM_VMEMMAP select HAVE_ARCH_KGDB select HAVE_ARCH_KMEMCHECK + select HAVE_ARCH_MMAP_RND_BITS if MMU + select HAVE_ARCH_MMAP_RND_COMPAT_BITS if MMU && COMPAT select HAVE_ARCH_SECCOMP_FILTER select HAVE_ARCH_SOFT_DIRTY if X86_64 select HAVE_ARCH_TRACEHOOK @@ -183,6 +185,20 @@ config HAVE_LATENCYTOP_SUPPORT config MMU def_bool y +config ARCH_MMAP_RND_BITS_MIN + default 28 if 64BIT + default 8 + +config ARCH_MMAP_RND_BITS_MAX + default 32 if 64BIT + default 16 + +config ARCH_MMAP_RND_COMPAT_BITS_MIN + default 8 + +config ARCH_MMAP_RND_COMPAT_BITS_MAX + default 16 + config SBUS bool diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c index 844b06d..647fecf 100644 --- a/arch/x86/mm/mmap.c +++ b/arch/x86/mm/mmap.c @@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void) { unsigned long rnd; - /* - * 8 bits of randomness in 32bit mmaps, 20 address space bits - * 28 bits of randomness in 64bit mmaps, 40 address space bits - */ if (mmap_is_ia32()) - rnd = (unsigned long)get_random_int() % (1<<8); +#ifdef CONFIG_COMPAT + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits); +#else + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits); +#endif else - rnd = (unsigned long)get_random_int() % (1<<28); + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits); return rnd << PAGE_SHIFT; }