* [PATCH] fsverity: reject FS_IOC_ENABLE_VERITY on mode 3 fds
@ 2023-04-06 21:51 Eric Biggers
2023-04-07 8:09 ` Christoph Hellwig
2023-04-11 8:56 ` Christian Brauner
0 siblings, 2 replies; 3+ messages in thread
From: Eric Biggers @ 2023-04-06 21:51 UTC (permalink / raw)
To: fsverity; +Cc: linux-fsdevel, stable, syzbot+51177e4144d764827c45
From: Eric Biggers <ebiggers@google.com>
Commit 56124d6c87fd ("fsverity: support enabling with tree block size <
PAGE_SIZE") changed FS_IOC_ENABLE_VERITY to use __kernel_read() to read
the file's data, instead of direct pagecache accesses.
An unintended consequence of this is that the
'WARN_ON_ONCE(!(file->f_mode & FMODE_READ))' in __kernel_read() became
reachable by fuzz tests. This happens if FS_IOC_ENABLE_VERITY is called
on a fd opened with access mode 3, which means "ioctl access only".
Arguably, FS_IOC_ENABLE_VERITY should work on ioctl-only fds. But
ioctl-only fds are a weird Linux extension that is rarely used and that
few people even know about. (The documentation for FS_IOC_ENABLE_VERITY
even specifically says it requires O_RDONLY.) It's probably not
worthwhile to make the ioctl internally open a new fd just to handle
this case. Thus, just reject the ioctl on such fds for now.
Fixes: 56124d6c87fd ("fsverity: support enabling with tree block size < PAGE_SIZE")
Reported-by: syzbot+51177e4144d764827c45@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=2281afcbbfa8fdb92f9887479cc0e4180f1c6b28
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
fs/verity/enable.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/fs/verity/enable.c b/fs/verity/enable.c
index bbec6f93172cf..fc4c50e5219dc 100644
--- a/fs/verity/enable.c
+++ b/fs/verity/enable.c
@@ -357,6 +357,13 @@ int fsverity_ioctl_enable(struct file *filp, const void __user *uarg)
err = file_permission(filp, MAY_WRITE);
if (err)
return err;
+ /*
+ * __kernel_read() is used while building the Merkle tree. So, we can't
+ * allow file descriptors that were opened for ioctl access only, using
+ * the special nonstandard access mode 3. O_RDONLY only, please!
+ */
+ if (!(filp->f_mode & FMODE_READ))
+ return -EBADF;
if (IS_APPEND(inode))
return -EPERM;
base-commit: dbd91ed3b5acb7acba2cac2d38e7aec57a5f1e96
--
2.40.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] fsverity: reject FS_IOC_ENABLE_VERITY on mode 3 fds
2023-04-06 21:51 [PATCH] fsverity: reject FS_IOC_ENABLE_VERITY on mode 3 fds Eric Biggers
@ 2023-04-07 8:09 ` Christoph Hellwig
2023-04-11 8:56 ` Christian Brauner
1 sibling, 0 replies; 3+ messages in thread
From: Christoph Hellwig @ 2023-04-07 8:09 UTC (permalink / raw)
To: Eric Biggers; +Cc: fsverity, linux-fsdevel, stable, syzbot+51177e4144d764827c45
Looks good:
Reviewed-by: Christoph Hellwig <hch@lst.de>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] fsverity: reject FS_IOC_ENABLE_VERITY on mode 3 fds
2023-04-06 21:51 [PATCH] fsverity: reject FS_IOC_ENABLE_VERITY on mode 3 fds Eric Biggers
2023-04-07 8:09 ` Christoph Hellwig
@ 2023-04-11 8:56 ` Christian Brauner
1 sibling, 0 replies; 3+ messages in thread
From: Christian Brauner @ 2023-04-11 8:56 UTC (permalink / raw)
To: Eric Biggers; +Cc: fsverity, linux-fsdevel, stable, syzbot+51177e4144d764827c45
On Thu, Apr 06, 2023 at 02:51:06PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
>
> Commit 56124d6c87fd ("fsverity: support enabling with tree block size <
> PAGE_SIZE") changed FS_IOC_ENABLE_VERITY to use __kernel_read() to read
> the file's data, instead of direct pagecache accesses.
>
> An unintended consequence of this is that the
> 'WARN_ON_ONCE(!(file->f_mode & FMODE_READ))' in __kernel_read() became
> reachable by fuzz tests. This happens if FS_IOC_ENABLE_VERITY is called
> on a fd opened with access mode 3, which means "ioctl access only".
>
> Arguably, FS_IOC_ENABLE_VERITY should work on ioctl-only fds. But
> ioctl-only fds are a weird Linux extension that is rarely used and that
> few people even know about. (The documentation for FS_IOC_ENABLE_VERITY
> even specifically says it requires O_RDONLY.) It's probably not
> worthwhile to make the ioctl internally open a new fd just to handle
> this case. Thus, just reject the ioctl on such fds for now.
>
> Fixes: 56124d6c87fd ("fsverity: support enabling with tree block size < PAGE_SIZE")
> Reported-by: syzbot+51177e4144d764827c45@syzkaller.appspotmail.com
> Link: https://syzkaller.appspot.com/bug?id=2281afcbbfa8fdb92f9887479cc0e4180f1c6b28
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
Looks good to me,
Reviewed-by: Christian Brauner <brauner@kernel.org>
^ permalink raw reply [flat|nested] 3+ messages in thread
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2023-04-06 21:51 [PATCH] fsverity: reject FS_IOC_ENABLE_VERITY on mode 3 fds Eric Biggers
2023-04-07 8:09 ` Christoph Hellwig
2023-04-11 8:56 ` Christian Brauner
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