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FAILED: patch "[PATCH] btrfs: file_remove_privs needs an exclusive lock in direct io" failed to apply to 6.1-stable tree

PATCH [email protected] 1 participant 1 message Oct 4, 2023

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The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 9af86694fd5d387992699ec99007ed374966ce9a
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2023100456-previous-tarantula-2517@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 9af86694fd5d387992699ec99007ed374966ce9a Mon Sep 17 00:00:00 2001
From: Bernd Schubert <[email protected]>
Date: Wed, 6 Sep 2023 17:59:03 +0200
Subject: [PATCH] btrfs: file_remove_privs needs an exclusive lock in direct io
write

This was noticed by Miklos that file_remove_privs might call into notify_change(), which requires to hold an exclusive lock. The problem exists in FUSE and btrfs. We can fix it without any additional helpers from VFS, in case the privileges would need to be dropped, change the lock type to be exclusive and redo the loop.

Fixes e9adabb9712e ("btrfs: use shared lock for direct writes within EOF")
CC Miklos Szeredi <[email protected]>
CC [email protected] # 5.15+
Reviewed-by Christoph Hellwig <[email protected]>
Signed-off-by Bernd Schubert <[email protected]>
Reviewed-by David Sterba <[email protected]>
Signed-off-by David Sterba <[email protected]>
fs/btrfs/file.c +14 -2
index 6edad7b9a5d3..e8726a83b649 100644
@@ -1466,8 +1466,13 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
 	if (iocb->ki_flags & IOCB_NOWAIT)
 		ilock_flags |= BTRFS_ILOCK_TRY;
 
-	/* If the write DIO is within EOF, use a shared lock */
-	if (iocb->ki_pos + iov_iter_count(from) <= i_size_read(inode))
+	/*
+	 * If the write DIO is within EOF, use a shared lock and also only if
+	 * security bits will likely not be dropped by file_remove_privs() called
+	 * from btrfs_write_check(). Either will need to be rechecked after the
+	 * lock was acquired.
+	 */
+	if (iocb->ki_pos + iov_iter_count(from) <= i_size_read(inode) && IS_NOSEC(inode))
 		ilock_flags |= BTRFS_ILOCK_SHARED;
 
 relock:

@@ -1475,6 +1480,13 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
 	if (err < 0)
 		return err;
 
+	/* Shared lock cannot be used with security bits set. */
+	if ((ilock_flags & BTRFS_ILOCK_SHARED) && !IS_NOSEC(inode)) {
+		btrfs_inode_unlock(BTRFS_I(inode), ilock_flags);
+		ilock_flags &= ~BTRFS_ILOCK_SHARED;
+		goto relock;
+	}
+
 	err = generic_write_checks(iocb, from);
 	if (err <= 0) {
 		btrfs_inode_unlock(BTRFS_I(inode), ilock_flags);