Re: [RFC][PATCH 2/4] slab: Detect negative size values and saturate

Kees Cook
2024-07-09T16:09:57Z

On Tue, Jul 09, 2024 at 08:57:55AM +0200, Przemek Kitszel wrote:
> On 7/8/24 21:18, Kees Cook wrote:
> > The allocator will already reject giant sizes seen from negative size
> > arguments, so this commit mainly services as an example for initial
> > type-based filtering. The size argument is checked for negative values
> > in signed arguments, saturating any if found instead of passing them on.
> >
> > For example, now the size is checked:
> >
> > Before:
> > /* %rdi unchecked */
> > 1eb: be c0 0c 00 00 mov $0xcc0,%esi
> > 1f0: e8 00 00 00 00 call 1f5 <do_SLAB_NEGATIVE+0x15>
> > 1f1: R_X86_64_PLT32 __kmalloc_noprof-0x4
> >
> > After:
> > 6d0: 48 63 c7 movslq %edi,%rax
> > 6d3: 85 ff test %edi,%edi
> > 6d5: be c0 0c 00 00 mov $0xcc0,%esi
> > 6da: 48 c7 c2 ff ff ff ff mov $0xffffffffffffffff,%rdx
> > 6e1: 48 0f 49 d0 cmovns %rax,%rdx
> > 6e5: 48 89 d7 mov %rdx,%rdi
> > 6e8: e8 00 00 00 00 call 6ed <do_SLAB_NEGATIVE+0x1d>
> > 6e9: R_X86_64_PLT32 __kmalloc_noprof-0x4
> >
> > Signed-off-by: Kees Cook <kees@kernel.org>
> > ---
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Pekka Enberg <penberg@kernel.org>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > Cc: Roman Gushchin <roman.gushchin@linux.dev>
> > Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> > Cc: linux-mm@kvack.org
> > ---
> > include/linux/slab.h | 19 ++++++++++++++++++-
> > 1 file changed, 18 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > index d99afce36098..7353756cbec6 100644
> > --- a/include/linux/slab.h
> > +++ b/include/linux/slab.h
> > @@ -684,7 +684,24 @@ static __always_inline __alloc_size(1) void *kmalloc_noprof(size_t size, gfp_t f
> > }
> > return __kmalloc_noprof(size, flags);
> > }
> > -#define kmalloc(...) alloc_hooks(kmalloc_noprof(__VA_ARGS__))
> > +#define kmalloc_sized(...) alloc_hooks(kmalloc_noprof(__VA_ARGS__))
> > +
> > +#define __size_force_positive(x) \
> > + ({ \
> > + typeof(__force_integral_expr(x)) __forced_val = \
> > + __force_integral_expr(x); \
> > + __forced_val < 0 ? SIZE_MAX : __forced_val; \
> > + })
> > +
> > +#define kmalloc(p, gfp) _Generic((p), \
> > + unsigned char: kmalloc_sized(__force_integral_expr(p), gfp), \
> > + unsigned short: kmalloc_sized(__force_integral_expr(p), gfp), \
> > + unsigned int: kmalloc_sized(__force_integral_expr(p), gfp), \
> > + unsigned long: kmalloc_sized(__force_integral_expr(p), gfp), \
> > + signed char: kmalloc_sized(__size_force_positive(p), gfp), \
> > + signed short: kmalloc_sized(__size_force_positive(p), gfp), \
> > + signed int: kmalloc_sized(__size_force_positive(p), gfp), \
> > + signed long: kmalloc_sized(__size_force_positive(p), gfp))
>
> I like this idea and series very much, thank you!

Thanks!

> What about bool?
> What about long long?

Ah yes, I will add these. LKP also found a weird one (a bitfield!) that
I'm fixing at the source:
https://lore.kernel.org/lkml/20240709154953.work.953-kees@kernel.org/

--
Kees Cook