| Message ID | 20240521122947.1080227-1-asmadeus@codewreck.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | 9p: add missing locking around taking dentry fid list | expand |
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.
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)
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(-)