linux-ext4 - [PATCH] ext4: fix a potential fiemap/page fault deadlock w/ inline

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Message-ID: <20181225055653.9382-1-tytso@mit.edu>
Date:   Tue, 25 Dec 2018 00:56:53 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     Ext4 Developers List <linux-ext4@...r.kernel.org>
CC:     Theodore Ts'o <tytso@....edu>, <stable@...nel.org>
Subject: [PATCH] ext4: fix a potential fiemap/page fault deadlock w/ inline_data

The ext4_inline_data_fiemap() function calls fiemap_fill_next_extent()
while still holding the xattr semaphore.  This is not necessary and it
triggers a circular lockdep warning.  This is because
fiemap_fill_next_extent() could trigger a page fault when it writes
into page which triggers a page fault.  If that page is mmaped from
the inline file in question, this could very well result in a
deadlock.

This problem can be reproduced using generic/519 with a file system
configuration which has the inline_data feature enabled.

Signed-off-by: Theodore Ts'o <tytso@....edu>
Cc: stable@...nel.org
---
 fs/ext4/inline.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
index 27373d88b5f0..56f6e1782d5f 100644
--- a/fs/ext4/inline.c
+++ b/fs/ext4/inline.c
@@ -1890,12 +1890,12 @@ int ext4_inline_data_fiemap(struct inode *inode,
 	physical += (char *)ext4_raw_inode(&iloc) - iloc.bh->b_data;
 	physical += offsetof(struct ext4_inode, i_block);

-	if (physical)
-		error = fiemap_fill_next_extent(fieinfo, start, physical,
-						inline_len, flags);
 	brelse(iloc.bh);
 out:
 	up_read(&EXT4_I(inode)->xattr_sem);
+	if (physical)
+		error = fiemap_fill_next_extent(fieinfo, start, physical,
+						inline_len, flags);
 	return (error < 0 ? error : 0);
 }

-- 
2.19.1