Fix a use-after-free on dentry's d_fsdata fid list when a thread lookups a fid through dentry while another thread unlinks it:
UAF thread: refcount_t: addition on 0; use-after-free. p9_fid_get linux/./include/net/9p/client.h:262 v9fs_fid_find+0x236/0x280 linux/fs/9p/fid.c:129 v9fs_fid_lookup_with_uid linux/fs/9p/fid.c:181 v9fs_fid_lookup+0xbf/0xc20 linux/fs/9p/fid.c:314 v9fs_vfs_getattr_dotl+0xf9/0x360 linux/fs/9p/vfs_inode_dotl.c:400 vfs_statx+0xdd/0x4d0 linux/fs/stat.c:248
Freed by: p9_client_clunk+0xb0/0xe0 linux/net/9p/client.c:1456 p9_fid_put linux/./include/net/9p/client.h:278 v9fs_dentry_release+0xb5/0x140 linux/fs/9p/vfs_dentry.c:55 v9fs_remove+0x38f/0x620 linux/fs/9p/vfs_inode.c:518 vfs_unlink+0x29a/0x810 linux/fs/namei.c:4335
The problem is that d_fsdata was not accessed under d_lock, because d_release() normally is only called once the dentry is otherwise no longer accessible but since we also call it explicitly in v9fs_remove that lock is required: move the hlist out of the dentry under lock then unref its fids once they are no longer accessible.
Fixes: 154372e67d40 ("fs/9p: fix create-unlink-getattr idiom") Cc: stable@vger.kernel.org Reported-by: Meysam Firouzi Reported-by: Amirmohammad Eftekhar Signed-off-by: Dominique Martinet asmadeus@codewreck.org --- fs/9p/vfs_dentry.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c index f16f73581634..01338d4c2d9e 100644 --- a/fs/9p/vfs_dentry.c +++ b/fs/9p/vfs_dentry.c @@ -48,12 +48,17 @@ static int v9fs_cached_dentry_delete(const struct dentry *dentry) static void v9fs_dentry_release(struct dentry *dentry) { struct hlist_node *p, *n; + struct hlist_head head;
p9_debug(P9_DEBUG_VFS, " dentry: %pd (%p)\n", dentry, dentry); - hlist_for_each_safe(p, n, (struct hlist_head *)&dentry->d_fsdata) + + spin_lock(&dentry->d_lock); + hlist_move_list((struct hlist_head *)&dentry->d_fsdata, &head); + spin_unlock(&dentry->d_lock); + + hlist_for_each_safe(p, n, &head) p9_fid_put(hlist_entry(p, struct p9_fid, dlist)); - dentry->d_fsdata = NULL; }
static int v9fs_lookup_revalidate(struct dentry *dentry, unsigned int flags)
On Tuesday, May 21, 2024 2:29:46 PM CEST Dominique Martinet wrote:
Fix a use-after-free on dentry's d_fsdata fid list when a thread lookups a fid through dentry while another thread unlinks it:
I guess that's "looks up". :)
UAF thread: refcount_t: addition on 0; use-after-free. p9_fid_get linux/./include/net/9p/client.h:262 v9fs_fid_find+0x236/0x280 linux/fs/9p/fid.c:129 v9fs_fid_lookup_with_uid linux/fs/9p/fid.c:181 v9fs_fid_lookup+0xbf/0xc20 linux/fs/9p/fid.c:314 v9fs_vfs_getattr_dotl+0xf9/0x360 linux/fs/9p/vfs_inode_dotl.c:400 vfs_statx+0xdd/0x4d0 linux/fs/stat.c:248
Freed by: p9_client_clunk+0xb0/0xe0 linux/net/9p/client.c:1456
That line number looks weird.
p9_fid_put linux/./include/net/9p/client.h:278 v9fs_dentry_release+0xb5/0x140 linux/fs/9p/vfs_dentry.c:55 v9fs_remove+0x38f/0x620 linux/fs/9p/vfs_inode.c:518 vfs_unlink+0x29a/0x810 linux/fs/namei.c:4335
The problem is that d_fsdata was not accessed under d_lock, because d_release() normally is only called once the dentry is otherwise no longer accessible but since we also call it explicitly in v9fs_remove that lock is required: move the hlist out of the dentry under lock then unref its fids once they are no longer accessible.
Fixes: 154372e67d40 ("fs/9p: fix create-unlink-getattr idiom") Cc: stable@vger.kernel.org Reported-by: Meysam Firouzi Reported-by: Amirmohammad Eftekhar Signed-off-by: Dominique Martinet asmadeus@codewreck.org
fs/9p/vfs_dentry.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c index f16f73581634..01338d4c2d9e 100644 --- a/fs/9p/vfs_dentry.c +++ b/fs/9p/vfs_dentry.c @@ -48,12 +48,17 @@ static int v9fs_cached_dentry_delete(const struct dentry *dentry) static void v9fs_dentry_release(struct dentry *dentry) { struct hlist_node *p, *n;
- struct hlist_head head;
p9_debug(P9_DEBUG_VFS, " dentry: %pd (%p)\n", dentry, dentry);
- hlist_for_each_safe(p, n, (struct hlist_head *)&dentry->d_fsdata)
- spin_lock(&dentry->d_lock);
- hlist_move_list((struct hlist_head *)&dentry->d_fsdata, &head);
- spin_unlock(&dentry->d_lock);
- hlist_for_each_safe(p, n, &head) p9_fid_put(hlist_entry(p, struct p9_fid, dlist));
- dentry->d_fsdata = NULL;
}
I'm not sure if that works out. So you are moving the list from dentry to a local variable. But if you look at v9fs_fid_find() [fs/9p/fid.c#123] it reads dentry->d_fsdata (twice) and holds it as local variable before taking a lock. So the lock in v9fs_fid_find() should happen earlier, no?
static int v9fs_lookup_revalidate(struct dentry *dentry, unsigned int flags)
Christian Schoenebeck wrote on Wed, May 22, 2024 at 04:35:19PM +0200:
Thanks for the review!
On Tuesday, May 21, 2024 2:29:46 PM CEST Dominique Martinet wrote:
Fix a use-after-free on dentry's d_fsdata fid list when a thread lookups a fid through dentry while another thread unlinks it:
I guess that's "looks up". :)
Err, I guess.
UAF thread: refcount_t: addition on 0; use-after-free. p9_fid_get linux/./include/net/9p/client.h:262 v9fs_fid_find+0x236/0x280 linux/fs/9p/fid.c:129 v9fs_fid_lookup_with_uid linux/fs/9p/fid.c:181 v9fs_fid_lookup+0xbf/0xc20 linux/fs/9p/fid.c:314 v9fs_vfs_getattr_dotl+0xf9/0x360 linux/fs/9p/vfs_inode_dotl.c:400 vfs_statx+0xdd/0x4d0 linux/fs/stat.c:248
Freed by: p9_client_clunk+0xb0/0xe0 linux/net/9p/client.c:1456
That line number looks weird.
I have a p9_fid_destroy there (as of a v6.9-rc5 tree); might have moved a bit though. Unfortunately it's inlined so the stack trace only has kfree() next which is why I cut the trace there; I don't think it really matters?
p9_fid_put linux/./include/net/9p/client.h:278 v9fs_dentry_release+0xb5/0x140 linux/fs/9p/vfs_dentry.c:55 v9fs_remove+0x38f/0x620 linux/fs/9p/vfs_inode.c:518 vfs_unlink+0x29a/0x810 linux/fs/namei.c:4335
The problem is that d_fsdata was not accessed under d_lock, because d_release() normally is only called once the dentry is otherwise no longer accessible but since we also call it explicitly in v9fs_remove that lock is required: move the hlist out of the dentry under lock then unref its fids once they are no longer accessible.
Fixes: 154372e67d40 ("fs/9p: fix create-unlink-getattr idiom") Cc: stable@vger.kernel.org Reported-by: Meysam Firouzi Reported-by: Amirmohammad Eftekhar Signed-off-by: Dominique Martinet asmadeus@codewreck.org
fs/9p/vfs_dentry.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c index f16f73581634..01338d4c2d9e 100644 --- a/fs/9p/vfs_dentry.c +++ b/fs/9p/vfs_dentry.c @@ -48,12 +48,17 @@ static int v9fs_cached_dentry_delete(const struct dentry *dentry) static void v9fs_dentry_release(struct dentry *dentry) { struct hlist_node *p, *n;
- struct hlist_head head;
p9_debug(P9_DEBUG_VFS, " dentry: %pd (%p)\n", dentry, dentry);
- hlist_for_each_safe(p, n, (struct hlist_head *)&dentry->d_fsdata)
- spin_lock(&dentry->d_lock);
- hlist_move_list((struct hlist_head *)&dentry->d_fsdata, &head);
- spin_unlock(&dentry->d_lock);
- hlist_for_each_safe(p, n, &head) p9_fid_put(hlist_entry(p, struct p9_fid, dlist));
- dentry->d_fsdata = NULL;
}
I'm not sure if that works out. So you are moving the list from dentry to a local variable. But if you look at v9fs_fid_find() [fs/9p/fid.c#123] it reads dentry->d_fsdata (twice) and holds it as local variable before taking a lock. So the lock in v9fs_fid_find() should happen earlier, no?
The comment still works -- if detry->d_fsdata is NULL then hlist_for_each_entry will stop short and not iterate over anything (it won't bug out), so that part is fine in my opinion.
What should be improved though is that if dentry->d_inode we can still look by inode even if there was a d_fsdata as log as fid wasn't found, e.g.: ----- diff --git a/fs/9p/fid.c b/fs/9p/fid.c index de009a33e0e2..c72825fb0ece 100644 --- a/fs/9p/fid.c +++ b/fs/9p/fid.c @@ -131,9 +131,9 @@ static struct p9_fid *v9fs_fid_find(struct dentry *dentry, kuid_t uid, int any) } } spin_unlock(&dentry->d_lock); - } else { - if (dentry->d_inode) - ret = v9fs_fid_find_inode(dentry->d_inode, false, uid, any); + } + if (!ret && dentry->d_inode) + ret = v9fs_fid_find_inode(dentry->d_inode, false, uid, any); }
return ret; ----
I don't think that has to be part of this commit though, the worst that can happen here is an extra lookup to server instead of a use after free; I'll send a separate patch for this.
On Wednesday, May 22, 2024 7:25:06 PM CEST Dominique Martinet wrote:
Christian Schoenebeck wrote on Wed, May 22, 2024 at 04:35:19PM +0200:
[...]
diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c index f16f73581634..01338d4c2d9e 100644 --- a/fs/9p/vfs_dentry.c +++ b/fs/9p/vfs_dentry.c @@ -48,12 +48,17 @@ static int v9fs_cached_dentry_delete(const struct dentry *dentry) static void v9fs_dentry_release(struct dentry *dentry) { struct hlist_node *p, *n;
- struct hlist_head head;
p9_debug(P9_DEBUG_VFS, " dentry: %pd (%p)\n", dentry, dentry);
- hlist_for_each_safe(p, n, (struct hlist_head *)&dentry->d_fsdata)
- spin_lock(&dentry->d_lock);
- hlist_move_list((struct hlist_head *)&dentry->d_fsdata, &head);
- spin_unlock(&dentry->d_lock);
- hlist_for_each_safe(p, n, &head) p9_fid_put(hlist_entry(p, struct p9_fid, dlist));
- dentry->d_fsdata = NULL;
}
I'm not sure if that works out. So you are moving the list from dentry to a local variable. But if you look at v9fs_fid_find() [fs/9p/fid.c#123] it reads dentry->d_fsdata (twice) and holds it as local variable before taking a lock. So the lock in v9fs_fid_find() should happen earlier, no?
The comment still works -- if detry->d_fsdata is NULL then hlist_for_each_entry will stop short and not iterate over anything (it won't bug out), so that part is fine in my opinion.
I meant the opposite: dentry->d_fsdata not being NULL. In this case v9fs_fid_find() takes a local copy of the list head pointer as `h` without taking a lock before.
Then v9fs_fid_find() takes the lock to run hlist_for_each_entry(), but at this point `h` could already point at garbage.
/Christian
Christian Schoenebeck wrote on Thu, May 23, 2024 at 10:34:14AM +0200:
The comment still works -- if detry->d_fsdata is NULL then hlist_for_each_entry will stop short and not iterate over anything (it won't bug out), so that part is fine in my opinion.
I meant the opposite: dentry->d_fsdata not being NULL.
I also meant that in the d_fsdata not being NULL branch, if d_fsdata turns out to be NULL when it is read under lock later.
In this case v9fs_fid_find() takes a local copy of the list head pointer as `h` without taking a lock before.
It doesn't, it takes &dentry->d_fsdata so the address of d_fsdata before the lock, but that address cannot change here (another thread cannot change the address of the dentry) ...(continuing below)
Then v9fs_fid_find() takes the lock to run hlist_for_each_entry(), but at this point `h` could already point at garbage.
... so *h (in practice, head->first in hlist_for_each_entry()) will properly contain the first node of the list under lock: either NULL if we just cleared it (at which point the loop won't iterate anything), or a new list if other items have been added meanwhile.
I really think it's safe, but I do agree that it's hard to read, happy to move the `h = &dentry->d_fsdata` inside the lock if you prefer -- it compiles to the same code for me (x86_64/gcc 13.2.0)
On Thursday, May 23, 2024 11:27:28 AM CEST Dominique Martinet wrote:
Christian Schoenebeck wrote on Thu, May 23, 2024 at 10:34:14AM +0200:
The comment still works -- if detry->d_fsdata is NULL then hlist_for_each_entry will stop short and not iterate over anything (it won't bug out), so that part is fine in my opinion.
I meant the opposite: dentry->d_fsdata not being NULL.
I also meant that in the d_fsdata not being NULL branch, if d_fsdata turns out to be NULL when it is read under lock later.
In this case v9fs_fid_find() takes a local copy of the list head pointer as `h` without taking a lock before.
It doesn't, it takes &dentry->d_fsdata so the address of d_fsdata before the lock, but that address cannot change here (another thread cannot change the address of the dentry) ...(continuing below)
Aaah right, I was missing the `&`, my bad!
Then v9fs_fid_find() takes the lock to run hlist_for_each_entry(), but at this point `h` could already point at garbage.
... so *h (in practice, head->first in hlist_for_each_entry()) will properly contain the first node of the list under lock: either NULL if we just cleared it (at which point the loop won't iterate anything), or a new list if other items have been added meanwhile.
Yeah, looks fine to me.
I really think it's safe, but I do agree that it's hard to read, happy to move the `h = &dentry->d_fsdata` inside the lock if you prefer -- it compiles to the same code for me (x86_64/gcc 13.2.0)
No need, you can add my RB. Thanks for the clarification!
Reviewed-by: Christian Schoenebeck linux_oss@crudebyte.com
Christian Schoenebeck wrote on Thu, May 23, 2024 at 12:05:44PM +0200:
I really think it's safe, but I do agree that it's hard to read, happy to move the `h = &dentry->d_fsdata` inside the lock if you prefer -- it compiles to the same code for me (x86_64/gcc 13.2.0)
No need, you can add my RB. Thanks for the clarification!
Reviewed-by: Christian Schoenebeck linux_oss@crudebyte.com
Thanks! I've fixed the typo in the commit message and queued it up in -next, will send this patch and the trace uninit fix to Linus early next week.
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Dominique Martinet