| Message ID | 20230711134623.12695-3-vbabka@suse.cz (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [1/2] mm/slub: remove redundant kasan_reset_tag() from freelist_ptr calculations | expand |
On Tue, Jul 11, 2023 at 03:46:24PM +0200, Vlastimil Babka wrote: > Commit d36a63a943e3 ("kasan, slub: fix more conflicts with > CONFIG_SLAB_FREELIST_HARDENED") has introduced kasan_reset_tags() to > freelist_ptr() encoding/decoding when CONFIG_SLAB_FREELIST_HARDENED is > enabled to resolve issues when passing tagged or untagged pointers > inconsistently would lead to incorrect calculations. > > Later, commit aa1ef4d7b3f6 ("kasan, mm: reset tags when accessing > metadata") made sure all pointers have tags reset regardless of > CONFIG_SLAB_FREELIST_HARDENED, because there was no other way to access > the freepointer metadata safely with hw tag-based KASAN. > > Therefore the kasan_reset_tag() usage in freelist_ptr_encode()/decode() > is now redundant, as all callers use kasan_reset_tag() unconditionally > when constructing ptr_addr. Remove the redundant calls and simplify the > code and remove obsolete comments. > > Also in freelist_ptr_encode() introduce an 'encoded' variable to make > the lines shorter and make it similar to the _decode() one. > > Signed-off-by: Vlastimil Babka <vbabka@suse.cz> Thanks, this is much more readable! Acked-by: Kees Cook <keescook@chromium.org>
On Tue, Jul 11, 2023 at 3:46 PM Vlastimil Babka <vbabka@suse.cz> wrote: > > Commit d36a63a943e3 ("kasan, slub: fix more conflicts with > CONFIG_SLAB_FREELIST_HARDENED") has introduced kasan_reset_tags() to > freelist_ptr() encoding/decoding when CONFIG_SLAB_FREELIST_HARDENED is > enabled to resolve issues when passing tagged or untagged pointers > inconsistently would lead to incorrect calculations. > > Later, commit aa1ef4d7b3f6 ("kasan, mm: reset tags when accessing > metadata") made sure all pointers have tags reset regardless of > CONFIG_SLAB_FREELIST_HARDENED, because there was no other way to access > the freepointer metadata safely with hw tag-based KASAN. > > Therefore the kasan_reset_tag() usage in freelist_ptr_encode()/decode() > is now redundant, as all callers use kasan_reset_tag() unconditionally > when constructing ptr_addr. Remove the redundant calls and simplify the > code and remove obsolete comments. > > Also in freelist_ptr_encode() introduce an 'encoded' variable to make > the lines shorter and make it similar to the _decode() one. > > Signed-off-by: Vlastimil Babka <vbabka@suse.cz> > --- > These 2 patches build on top of: > https://lore.kernel.org/all/20230704135834.3884421-1-matteorizzo@google.com/ > > mm/slub.c | 22 ++++++---------------- > 1 file changed, 6 insertions(+), 16 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index f8cc47eff742..07edad305512 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -374,22 +374,14 @@ typedef struct { unsigned long v; } freeptr_t; > static inline freeptr_t freelist_ptr_encode(const struct kmem_cache *s, > void *ptr, unsigned long ptr_addr) > { > + unsigned long encoded; > + > #ifdef CONFIG_SLAB_FREELIST_HARDENED > - /* > - * When CONFIG_KASAN_SW/HW_TAGS is enabled, ptr_addr might be tagged. > - * Normally, this doesn't cause any issues, as both set_freepointer() > - * and get_freepointer() are called with a pointer with the same tag. > - * However, there are some issues with CONFIG_SLUB_DEBUG code. For > - * example, when __free_slub() iterates over objects in a cache, it > - * passes untagged pointers to check_object(). check_object() in turns > - * calls get_freepointer() with an untagged pointer, which causes the > - * freepointer to be restored incorrectly. > - */ > - return (freeptr_t){.v = (unsigned long)ptr ^ s->random ^ > - swab((unsigned long)kasan_reset_tag((void *)ptr_addr))}; > + encoded = (unsigned long)ptr ^ s->random ^ swab(ptr_addr); > #else > - return (freeptr_t){.v = (unsigned long)ptr}; > + encoded = (unsigned long)ptr; > #endif > + return (freeptr_t){.v = encoded}; > } > > static inline void *freelist_ptr_decode(const struct kmem_cache *s, > @@ -398,9 +390,7 @@ static inline void *freelist_ptr_decode(const struct kmem_cache *s, > void *decoded; > > #ifdef CONFIG_SLAB_FREELIST_HARDENED > - /* See the comment in freelist_ptr_encode */ > - decoded = (void *)(ptr.v ^ s->random ^ > - swab((unsigned long)kasan_reset_tag((void *)ptr_addr))); > + decoded = (void *)(ptr.v ^ s->random ^ swab(ptr_addr)); > #else > decoded = (void *)ptr.v; > #endif > -- > 2.41.0 Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com> Thanks!
diff --git a/mm/slub.c b/mm/slub.c index f8cc47eff742..07edad305512 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -374,22 +374,14 @@ typedef struct { unsigned long v; } freeptr_t; static inline freeptr_t freelist_ptr_encode(const struct kmem_cache *s, void *ptr, unsigned long ptr_addr) { + unsigned long encoded; + #ifdef CONFIG_SLAB_FREELIST_HARDENED - /* - * When CONFIG_KASAN_SW/HW_TAGS is enabled, ptr_addr might be tagged. - * Normally, this doesn't cause any issues, as both set_freepointer() - * and get_freepointer() are called with a pointer with the same tag. - * However, there are some issues with CONFIG_SLUB_DEBUG code. For - * example, when __free_slub() iterates over objects in a cache, it - * passes untagged pointers to check_object(). check_object() in turns - * calls get_freepointer() with an untagged pointer, which causes the - * freepointer to be restored incorrectly. - */ - return (freeptr_t){.v = (unsigned long)ptr ^ s->random ^ - swab((unsigned long)kasan_reset_tag((void *)ptr_addr))}; + encoded = (unsigned long)ptr ^ s->random ^ swab(ptr_addr); #else - return (freeptr_t){.v = (unsigned long)ptr}; + encoded = (unsigned long)ptr; #endif + return (freeptr_t){.v = encoded}; } static inline void *freelist_ptr_decode(const struct kmem_cache *s, @@ -398,9 +390,7 @@ static inline void *freelist_ptr_decode(const struct kmem_cache *s, void *decoded; #ifdef CONFIG_SLAB_FREELIST_HARDENED - /* See the comment in freelist_ptr_encode */ - decoded = (void *)(ptr.v ^ s->random ^ - swab((unsigned long)kasan_reset_tag((void *)ptr_addr))); + decoded = (void *)(ptr.v ^ s->random ^ swab(ptr_addr)); #else decoded = (void *)ptr.v; #endif
Commit d36a63a943e3 ("kasan, slub: fix more conflicts with CONFIG_SLAB_FREELIST_HARDENED") has introduced kasan_reset_tags() to freelist_ptr() encoding/decoding when CONFIG_SLAB_FREELIST_HARDENED is enabled to resolve issues when passing tagged or untagged pointers inconsistently would lead to incorrect calculations. Later, commit aa1ef4d7b3f6 ("kasan, mm: reset tags when accessing metadata") made sure all pointers have tags reset regardless of CONFIG_SLAB_FREELIST_HARDENED, because there was no other way to access the freepointer metadata safely with hw tag-based KASAN. Therefore the kasan_reset_tag() usage in freelist_ptr_encode()/decode() is now redundant, as all callers use kasan_reset_tag() unconditionally when constructing ptr_addr. Remove the redundant calls and simplify the code and remove obsolete comments. Also in freelist_ptr_encode() introduce an 'encoded' variable to make the lines shorter and make it similar to the _decode() one. Signed-off-by: Vlastimil Babka <vbabka@suse.cz> --- These 2 patches build on top of: https://lore.kernel.org/all/20230704135834.3884421-1-matteorizzo@google.com/ mm/slub.c | 22 ++++++---------------- 1 file changed, 6 insertions(+), 16 deletions(-)