[PATCH AUTOSEL 6.10 7/7] vfs: Fix potential circular locking through setxattr() and removexattr()

Sasha Levin
2024-07-30T12:45:37Z

From: David Howells <dhowells@redhat.com>

[ Upstream commit c3a5e3e872f3688ae0dc57bb78ca633921d96a91 ]

When using cachefiles, lockdep may emit something similar to the circular
locking dependency notice below. The problem appears to stem from the
following:

(1) Cachefiles manipulates xattrs on the files in its cache when called
from ->writepages().

(2) The setxattr() and removexattr() system call handlers get the name
(and value) from userspace after taking the sb_writers lock, putting
accesses of the vma->vm_lock and mm->mmap_lock inside of that.

(3) The afs filesystem uses a per-inode lock to prevent multiple
revalidation RPCs and in writeback vs truncate to prevent parallel
operations from deadlocking against the server on one side and local
page locks on the other.

Fix this by moving the getting of the name and value in {get,remove}xattr()
outside of the sb_writers lock. This also has the minor benefits that we
don't need to reget these in the event of a retry and we never try to take
the sb_writers lock in the event we can't pull the name and value into the
kernel.

Alternative approaches that might fix this include moving the dispatch of a
write to the cache off to a workqueue or trying to do without the
validation lock in afs. Note that this might also affect other filesystems
that use netfslib and/or cachefiles.

======================================================
WARNING: possible circular locking dependency detected
6.10.0-build2+ #956 Not tainted
------------------------------------------------------
fsstress/6050 is trying to acquire lock:
ffff888138fd82f0 (mapping.invalidate_lock#3){++++}-{3:3}, at: filemap_fault+0x26e/0x8b0

but task is already holding lock:
ffff888113f26d18 (&vma->vm_lock->lock){++++}-{3:3}, at: lock_vma_under_rcu+0x165/0x250

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #4 (&vma->vm_lock->lock){++++}-{3:3}:
__lock_acquire+0xaf0/0xd80
lock_acquire.part.0+0x103/0x280
down_write+0x3b/0x50
vma_start_write+0x6b/0xa0
vma_link+0xcc/0x140
insert_vm_struct+0xb7/0xf0
alloc_bprm+0x2c1/0x390
kernel_execve+0x65/0x1a0
call_usermodehelper_exec_async+0x14d/0x190
ret_from_fork+0x24/0x40
ret_from_fork_asm+0x1a/0x30

-> #3 (&mm->mmap_lock){++++}-{3:3}:
__lock_acquire+0xaf0/0xd80
lock_acquire.part.0+0x103/0x280
__might_fault+0x7c/0xb0
strncpy_from_user+0x25/0x160
removexattr+0x7f/0x100
__do_sys_fremovexattr+0x7e/0xb0
do_syscall_64+0x9f/0x100
entry_SYSCALL_64_after_hwframe+0x76/0x7e

-> #2 (sb_writers#14){.+.+}-{0:0}:
__lock_acquire+0xaf0/0xd80
lock_acquire.part.0+0x103/0x280
percpu_down_read+0x3c/0x90
vfs_iocb_iter_write+0xe9/0x1d0
__cachefiles_write+0x367/0x430
cachefiles_issue_write+0x299/0x2f0
netfs_advance_write+0x117/0x140
netfs_write_folio.isra.0+0x5ca/0x6e0
netfs_writepages+0x230/0x2f0
afs_writepages+0x4d/0x70
do_writepages+0x1e8/0x3e0
filemap_fdatawrite_wbc+0x84/0xa0
__filemap_fdatawrite_range+0xa8/0xf0
file_write_and_wait_range+0x59/0x90
afs_release+0x10f/0x270
__fput+0x25f/0x3d0
__do_sys_close+0x43/0x70
do_syscall_64+0x9f/0x100
entry_SYSCALL_64_after_hwframe+0x76/0x7e

-> #1 (&vnode->validate_lock){++++}-{3:3}:
__lock_acquire+0xaf0/0xd80
lock_acquire.part.0+0x103/0x280
down_read+0x95/0x200
afs_writepages+0x37/0x70
do_writepages+0x1e8/0x3e0
filemap_fdatawrite_wbc+0x84/0xa0
filemap_invalidate_inode+0x167/0x1e0
netfs_unbuffered_write_iter+0x1bd/0x2d0
vfs_write+0x22e/0x320
ksys_write+0xbc/0x130
do_syscall_64+0x9f/0x100
entry_SYSCALL_64_after_hwframe+0x76/0x7e

-> #0 (mapping.invalidate_lock#3){++++}-{3:3}:
check_noncircular+0x119/0x160
check_prev_add+0x195/0x430
__lock_acquire+0xaf0/0xd80
lock_acquire.part.0+0x103/0x280
down_read+0x95/0x200
filemap_fault+0x26e/0x8b0
__do_fault+0x57/0xd0
do_pte_missing+0x23b/0x320
__handle_mm_fault+0x2d4/0x320
handle_mm_fault+0x14f/0x260
do_user_addr_fault+0x2a2/0x500
exc_page_fault+0x71/0x90
asm_exc_page_fault+0x22/0x30

other info that might help us debug this:

Chain exists of:
mapping.invalidate_lock#3 --> &mm->mmap_lock --> &vma->vm_lock->lock

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
rlock(&vma->vm_lock->lock);
lock(&mm->mmap_lock);
lock(&vma->vm_lock->lock);
rlock(mapping.invalidate_lock#3);

*** DEADLOCK ***

1 lock held by fsstress/6050:
#0: ffff888113f26d18 (&vma->vm_lock->lock){++++}-{3:3}, at: lock_vma_under_rcu+0x165/0x250

stack backtrace:
CPU: 0 PID: 6050 Comm: fsstress Not tainted 6.10.0-build2+ #956
Hardware name: ASUS All Series/H97-PLUS, BIOS 2306 10/09/2014
Call Trace:
<TASK>
dump_stack_lvl+0x57/0x80
check_noncircular+0x119/0x160
? queued_spin_lock_slowpath+0x4be/0x510
? __pfx_check_noncircular+0x10/0x10
? __pfx_queued_spin_lock_slowpath+0x10/0x10
? mark_lock+0x47/0x160
? init_chain_block+0x9c/0xc0
? add_chain_block+0x84/0xf0
check_prev_add+0x195/0x430
__lock_acquire+0xaf0/0xd80
? __pfx___lock_acquire+0x10/0x10
? __lock_release.isra.0+0x13b/0x230
lock_acquire.part.0+0x103/0x280
? filemap_fault+0x26e/0x8b0
? __pfx_lock_acquire.part.0+0x10/0x10
? rcu_is_watching+0x34/0x60
? lock_acquire+0xd7/0x120
down_read+0x95/0x200
? filemap_fault+0x26e/0x8b0
? __pfx_down_read+0x10/0x10
? __filemap_get_folio+0x25/0x1a0
filemap_fault+0x26e/0x8b0
? __pfx_filemap_fault+0x10/0x10
? find_held_lock+0x7c/0x90
? __pfx___lock_release.isra.0+0x10/0x10
? __pte_offset_map+0x99/0x110
__do_fault+0x57/0xd0
do_pte_missing+0x23b/0x320
__handle_mm_fault+0x2d4/0x320
? __pfx___handle_mm_fault+0x10/0x10
handle_mm_fault+0x14f/0x260
do_user_addr_fault+0x2a2/0x500
exc_page_fault+0x71/0x90
asm_exc_page_fault+0x22/0x30

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/2136178.1721725194@warthog.procyon.org.uk
cc: Alexander Viro <viro@zeniv.linux.org.uk>
cc: Christian Brauner <brauner@kernel.org>
cc: Jan Kara <jack@suse.cz>
cc: Jeff Layton <jlayton@kernel.org>
cc: Gao Xiang <xiang@kernel.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: netfs@lists.linux.dev
cc: linux-erofs@lists.ozlabs.org
cc: linux-fsdevel@vger.kernel.org
[brauner: fix minor issues]
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/xattr.c | 91 ++++++++++++++++++++++++++++--------------------------
1 file changed, 48 insertions(+), 43 deletions(-)

diff --git a/fs/xattr.c b/fs/xattr.c
index f8b643f91a981..7672ce5486c53 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -630,10 +630,9 @@ int do_setxattr(struct mnt_idmap *idmap, struct dentry *dentry,
ctx->kvalue, ctx->size, ctx->flags);
}

-static long
-setxattr(struct mnt_idmap *idmap, struct dentry *d,
- const char __user *name, const void __user *value, size_t size,
- int flags)
+static int path_setxattr(const char __user *pathname,
+ const char __user *name, const void __user *value,
+ size_t size, int flags, unsigned int lookup_flags)
{
struct xattr_name kname;
struct xattr_ctx ctx = {
@@ -643,33 +642,20 @@ setxattr(struct mnt_idmap *idmap, struct dentry *d,
.kname = &kname,
.flags = flags,
};
+ struct path path;
int error;

error = setxattr_copy(name, &ctx);
if (error)
return error;

- error = do_setxattr(idmap, d, &ctx);
-
- kvfree(ctx.kvalue);
- return error;
-}
-
-static int path_setxattr(const char __user *pathname,
- const char __user *name, const void __user *value,
- size_t size, int flags, unsigned int lookup_flags)
-{
- struct path path;
- int error;
-
retry:
error = user_path_at(AT_FDCWD, pathname, lookup_flags, &path);
if (error)
- return error;
+ goto out;
error = mnt_want_write(path.mnt);
if (!error) {
- error = setxattr(mnt_idmap(path.mnt), path.dentry, name,
- value, size, flags);
+ error = do_setxattr(mnt_idmap(path.mnt), path.dentry, &ctx);
mnt_drop_write(path.mnt);
}
path_put(&path);
@@ -677,6 +663,9 @@ static int path_setxattr(const char __user *pathname,
lookup_flags |= LOOKUP_REVAL;
goto retry;
}
+
+out:
+ kvfree(ctx.kvalue);
return error;
}

@@ -697,20 +686,32 @@ SYSCALL_DEFINE5(lsetxattr, const char __user *, pathname,
SYSCALL_DEFINE5(fsetxattr, int, fd, const char __user *, name,
const void __user *,value, size_t, size, int, flags)
{
- struct fd f = fdget(fd);
- int error = -EBADF;
+ struct xattr_name kname;
+ struct xattr_ctx ctx = {
+ .cvalue = value,
+ .kvalue = NULL,
+ .size = size,
+ .kname = &kname,
+ .flags = flags,
+ };
+ int error;

+ CLASS(fd, f)(fd);
if (!f.file)
- return error;
+ return -EBADF;
+
audit_file(f.file);
+ error = setxattr_copy(name, &ctx);
+ if (error)
+ return error;
+
error = mnt_want_write_file(f.file);
if (!error) {
- error = setxattr(file_mnt_idmap(f.file),
- f.file->f_path.dentry, name,
- value, size, flags);
+ error = do_setxattr(file_mnt_idmap(f.file),
+ f.file->f_path.dentry, &ctx);
mnt_drop_write_file(f.file);
}
- fdput(f);
+ kvfree(ctx.kvalue);
return error;
}

@@ -899,9 +900,17 @@ SYSCALL_DEFINE3(flistxattr, int, fd, char __user *, list, size_t, size)
* Extended attribute REMOVE operations
*/
static long
-removexattr(struct mnt_idmap *idmap, struct dentry *d,
- const char __user *name)
+removexattr(struct mnt_idmap *idmap, struct dentry *d, const char *name)
{
+ if (is_posix_acl_xattr(name))
+ return vfs_remove_acl(idmap, d, name);
+ return vfs_removexattr(idmap, d, name);
+}
+
+static int path_removexattr(const char __user *pathname,
+ const char __user *name, unsigned int lookup_flags)
+{
+ struct path path;
int error;
char kname[XATTR_NAME_MAX + 1];

@@ -910,25 +919,13 @@ removexattr(struct mnt_idmap *idmap, struct dentry *d,
error = -ERANGE;
if (error < 0)
return error;
-
- if (is_posix_acl_xattr(kname))
- return vfs_remove_acl(idmap, d, kname);
-
- return vfs_removexattr(idmap, d, kname);
-}
-
-static int path_removexattr(const char __user *pathname,
- const char __user *name, unsigned int lookup_flags)
-{
- struct path path;
- int error;
retry:
error = user_path_at(AT_FDCWD, pathname, lookup_flags, &path);
if (error)
return error;
error = mnt_want_write(path.mnt);
if (!error) {
- error = removexattr(mnt_idmap(path.mnt), path.dentry, name);
+ error = removexattr(mnt_idmap(path.mnt), path.dentry, kname);
mnt_drop_write(path.mnt);
}
path_put(&path);
@@ -954,15 +951,23 @@ SYSCALL_DEFINE2(lremovexattr, const char __user *, pathname,
SYSCALL_DEFINE2(fremovexattr, int, fd, const char __user *, name)
{
struct fd f = fdget(fd);
+ char kname[XATTR_NAME_MAX + 1];
int error = -EBADF;

if (!f.file)
return error;
audit_file(f.file);
+
+ error = strncpy_from_user(kname, name, sizeof(kname));
+ if (error == 0 || error == sizeof(kname))
+ error = -ERANGE;
+ if (error < 0)
+ return error;
+
error = mnt_want_write_file(f.file);
if (!error) {
error = removexattr(file_mnt_idmap(f.file),
- f.file->f_path.dentry, name);
+ f.file->f_path.dentry, kname);
mnt_drop_write_file(f.file);
}
fdput(f);
--
2.43.0