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fs-verity: use smp_load_acquire() for ->i_verity_info

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Normally smp_store_release() or cmpxchg_release() is paired with
smp_load_acquire().  Sometimes smp_load_acquire() can be replaced with
the more lightweight READ_ONCE().  However, for this to be safe, all the
published memory must only be accessed in a way that involves the
pointer itself.  This may not be the case if allocating the object also
involves initializing a static or global variable, for example.

fsverity_info::tree_params.hash_alg->tfm is a crypto_ahash object that's
internal to and is allocated by the crypto subsystem.  So by using
READ_ONCE() for ->i_verity_info, we're relying on internal
implementation details of the crypto subsystem.

Remove this fragile assumption by using smp_load_acquire() instead.

Also fix the cmpxchg logic to correctly execute an ACQUIRE barrier when
losing the cmpxchg race, since cmpxchg doesn't guarantee a memory
barrier on failure.

(Note: I haven't seen any real-world problems here.  This change is just
fixing the code to be guaranteed correct and less fragile.)

Fixes: fd2d1acfca ("fs-verity: add the hook for file ->open()")
Link: https://lore.kernel.org/r/20200721225920.114347-6-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
This commit is contained in:
Eric Biggers 2020-07-21 15:59:20 -07:00
parent ba47d845d7
commit f3db0bed45
2 changed files with 19 additions and 5 deletions
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15
fs/verity/open.c
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@ -221,11 +221,20 @@ struct fsverity_info *fsverity_create_info(const struct inode *inode,
void fsverity_set_info(struct inode *inode, struct fsverity_info *vi)
{
/*
* Multiple processes may race to set ->i_verity_info, so use cmpxchg.
* This pairs with the READ_ONCE() in fsverity_get_info().
* Multiple tasks may race to set ->i_verity_info, so use
* cmpxchg_release(). This pairs with the smp_load_acquire() in
* fsverity_get_info(). I.e., here we publish ->i_verity_info with a
* RELEASE barrier so that other tasks can ACQUIRE it.
*/
if (cmpxchg(&inode->i_verity_info, NULL, vi) != NULL)
if (cmpxchg_release(&inode->i_verity_info, NULL, vi) != NULL) {
/* Lost the race, so free the fsverity_info we allocated. */
fsverity_free_info(vi);
/*
* Afterwards, the caller may access ->i_verity_info directly,
* so make sure to ACQUIRE the winning fsverity_info.
*/
(void)fsverity_get_info(inode);
}
}
void fsverity_free_info(struct fsverity_info *vi)

9
include/linux/fsverity.h
View File

@ -115,8 +115,13 @@ struct fsverity_operations {
static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
{
/* pairs with the cmpxchg() in fsverity_set_info() */
return READ_ONCE(inode->i_verity_info);
/*
* Pairs with the cmpxchg_release() in fsverity_set_info().
* I.e., another task may publish ->i_verity_info concurrently,
* executing a RELEASE barrier. We need to use smp_load_acquire() here
* to safely ACQUIRE the memory the other task published.
*/
return smp_load_acquire(&inode->i_verity_info);
}
/* enable.c */