In a couple of places in the slub memory allocator, the code uses
"s->offset" as a check to see if the free pointer is put right after the
object. That check is no longer true with commit 3202fa62fb43 ("slub:
relocate freelist pointer to middle of object").
As a result, echoing "1" into the validate sysfs file, e.g. of dentry,
may cause a bunch of "Freepointer corrupt" error reports like the
following to appear with the system in panic afterwards.
[ 38.579769] =============================================================================
[ 38.580845] BUG dentry(666:pmcd.service) (Tainted: G B): Freepointer corrupt
[ 38.581948] -----------------------------------------------------------------------------
To fix it, use the check "s->offset == s->inuse" in the new helper
function freeptr_after_object() instead. Also add another helper function
get_info_end() to return the end of info block (inuse + free pointer
if not overlapping with object).
Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
Signed-off-by: Waiman Long <[email protected]>
---
mm/slub.c | 37 ++++++++++++++++++++++---------------
1 file changed, 22 insertions(+), 15 deletions(-)
diff --git a/mm/slub.c b/mm/slub.c
index 0e736d66bb42..68f1b4b1c309 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -551,15 +551,29 @@ static void print_section(char *level, char *text, u8 *addr,
metadata_access_disable();
}
+static inline bool freeptr_after_object(struct kmem_cache *s)
+{
+ return s->offset == s->inuse;
+}
+
+/*
+ * Return offset of the end of info block which is inuse + free pointer if
+ * not overlapping with object.
+ */
+static inline unsigned int get_info_end(struct kmem_cache *s)
+{
+ if (freeptr_after_object(s))
+ return s->inuse + sizeof(void *);
+ else
+ return s->inuse;
+}
+
static struct track *get_track(struct kmem_cache *s, void *object,
enum track_item alloc)
{
struct track *p;
- if (s->offset)
- p = object + s->offset + sizeof(void *);
- else
- p = object + s->inuse;
+ p = object + get_info_end(s);
return p + alloc;
}
@@ -693,10 +707,7 @@ static void print_trailer(struct kmem_cache *s, struct page *page, u8 *p)
print_section(KERN_ERR, "Redzone ", p + s->object_size,
s->inuse - s->object_size);
- if (s->offset)
- off = s->offset + sizeof(void *);
- else
- off = s->inuse;
+ off = get_info_end(s);
if (s->flags & SLAB_STORE_USER)
off += 2 * sizeof(struct track);
@@ -790,7 +801,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct page *page,
* object address
* Bytes of the object to be managed.
* If the freepointer may overlay the object then the free
- * pointer is the first word of the object.
+ * pointer is at the middle of the object.
*
* Poisoning uses 0x6b (POISON_FREE) and the last byte is
* 0xa5 (POISON_END)
@@ -824,11 +835,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct page *page,
static int check_pad_bytes(struct kmem_cache *s, struct page *page, u8 *p)
{
- unsigned long off = s->inuse; /* The end of info */
-
- if (s->offset)
- /* Freepointer is placed after the object. */
- off += sizeof(void *);
+ unsigned long off = get_info_end(s); /* The end of info */
if (s->flags & SLAB_STORE_USER)
/* We also have user information there */
@@ -915,7 +922,7 @@ static int check_object(struct kmem_cache *s, struct page *page,
check_pad_bytes(s, page, p);
}
- if (!s->offset && val == SLUB_RED_ACTIVE)
+ if (!freeptr_after_object(s) && val == SLUB_RED_ACTIVE)
/*
* Object and freepointer overlap. Cannot check
* freepointer while object is allocated.
--
2.18.1
On Mon, Apr 27, 2020 at 10:08:22AM -0400, Waiman Long wrote:
> In a couple of places in the slub memory allocator, the code uses
> "s->offset" as a check to see if the free pointer is put right after the
> object. That check is no longer true with commit 3202fa62fb43 ("slub:
> relocate freelist pointer to middle of object").
>
> As a result, echoing "1" into the validate sysfs file, e.g. of dentry,
> may cause a bunch of "Freepointer corrupt" error reports like the
> following to appear with the system in panic afterwards.
>
> [ 38.579769] =============================================================================
> [ 38.580845] BUG dentry(666:pmcd.service) (Tainted: G B): Freepointer corrupt
> [ 38.581948] -----------------------------------------------------------------------------
>
> To fix it, use the check "s->offset == s->inuse" in the new helper
> function freeptr_after_object() instead. Also add another helper function
> get_info_end() to return the end of info block (inuse + free pointer
> if not overlapping with object).
>
> Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
> Signed-off-by: Waiman Long <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
On Mon, 27 Apr 2020, Waiman Long wrote:
>
> To fix it, use the check "s->offset == s->inuse" in the new helper
> function freeptr_after_object() instead. Also add another helper function
> get_info_end() to return the end of info block (inuse + free pointer
> if not overlapping with object).
>
> Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
> Signed-off-by: Waiman Long <[email protected]>
> ---
> mm/slub.c | 37 ++++++++++++++++++++++---------------
> 1 file changed, 22 insertions(+), 15 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 0e736d66bb42..68f1b4b1c309 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -551,15 +551,29 @@ static void print_section(char *level, char *text, u8 *addr,
> metadata_access_disable();
> }
>
> +static inline bool freeptr_after_object(struct kmem_cache *s)
bool freeptr_outside_of_object()?
> +{
> + return s->offset == s->inuse;
s->offset >= s->inuse?
There may be a redzone after the object.
> +static inline unsigned int get_info_end(struct kmem_cache *s)
static inline track_offset()?
On 4/27/20 12:10 PM, Christopher Lameter wrote:
> On Mon, 27 Apr 2020, Waiman Long wrote:
>
>> To fix it, use the check "s->offset == s->inuse" in the new helper
>> function freeptr_after_object() instead. Also add another helper function
>> get_info_end() to return the end of info block (inuse + free pointer
>> if not overlapping with object).
>>
>> Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
>> Signed-off-by: Waiman Long <[email protected]>
>> ---
>> mm/slub.c | 37 ++++++++++++++++++++++---------------
>> 1 file changed, 22 insertions(+), 15 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 0e736d66bb42..68f1b4b1c309 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -551,15 +551,29 @@ static void print_section(char *level, char *text, u8 *addr,
>> metadata_access_disable();
>> }
>>
>> +static inline bool freeptr_after_object(struct kmem_cache *s)
> bool freeptr_outside_of_object()?
>
I can change to that name. It doesn't really matter to me.
>> +{
>> + return s->offset == s->inuse;
> s->offset >= s->inuse?
>
> There may be a redzone after the object.
>
Technically inuse is object + red zone. According to calculate_sizes():
??????? s->inuse = size;
??????? if (((flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) ||
??????????????? s->ctor)) {
??????????????? /*
???????????????? * Relocate free pointer after the object if it is not
???????????????? * permitted to overwrite the first word of the object on
???????????????? * kmem_cache_free.
???????????????? *
???????????????? * This is the case if we do RCU, have a constructor or
???????????????? * destructor or are poisoning the objects.
???????????????? */
??????????????? s->offset = size;
??????????????? size += sizeof(void *);
So (s->offset == s->inuse) when the free pointer is outside of the object.
>> +static inline unsigned int get_info_end(struct kmem_cache *s)
> static inline track_offset()?
>
The main reason why I don't use that is because there is a track data
structure in slub. There are functions name get_track() and set_track().
I don't want to confuse with them.
Cheers,
Longman