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

[Przemek Kitszel]

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@xxxxxxxxxx>
> ---
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> Cc: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> ---
>   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!

What about bool?
What about long long?
(by this commit one will get a rather easy to parse compile error, but
the next one will obscure it a bit)

Consider the following correct (albeit somewhat weird) code:
	/* header */
	char *state;

	/* .c impl, init part */
	bool needs_state = some_expr();
	state = kmalloc(needs_state, GFP_KERNEL);

	/* .c, other part */
	if (ZERO_OR_NULL_PTR(state))
		return _EARLY;
	*state = state_machine_action(*state);

>   
>   #define kmem_buckets_alloc(_b, _size, _flags)	\
>   	alloc_hooks(__kmalloc_node_noprof(PASS_BUCKET_PARAMS(_size, _b), _flags, NUMA_NO_NODE))