linux-kernel - [PATCH 3.16 054/148] jbd2: Fix possible overflow in jbd2_log_space

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Message-ID: <lsq.1581185940.155649612@decadent.org.uk>
Date:   Sat, 08 Feb 2020 18:19:53 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Theodore Ts'o" <tytso@....edu>, "Jan Kara" <jack@...e.cz>
Subject: [PATCH 3.16 054/148] jbd2: Fix possible overflow in
 jbd2_log_space_left()

3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jan Kara <jack@...e.cz>

commit add3efdd78b8a0478ce423bb9d4df6bd95e8b335 upstream.

When number of free space in the journal is very low, the arithmetic in
jbd2_log_space_left() could underflow resulting in very high number of
free blocks and thus triggering assertion failure in transaction commit
code complaining there's not enough space in the journal:

J_ASSERT(journal->j_free > 1);

Properly check for the low number of free blocks.

Reviewed-by: Theodore Ts'o <tytso@....edu>
Signed-off-by: Jan Kara <jack@...e.cz>
Link: https://lore.kernel.org/r/20191105164437.32602-1-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@....edu>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 include/linux/jbd2.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1340,7 +1340,7 @@ static inline int jbd2_space_needed(jour
 static inline unsigned long jbd2_log_space_left(journal_t *journal)
 {
 	/* Allow for rounding errors */
-	unsigned long free = journal->j_free - 32;
+	long free = journal->j_free - 32;
 
 	if (journal->j_committing_transaction) {
 		unsigned long committing = atomic_read(&journal->
@@ -1349,7 +1349,7 @@ static inline unsigned long jbd2_log_spa
 		/* Transaction + control blocks */
 		free -= committing + (committing >> JBD2_CONTROL_BLOCKS_SHIFT);
 	}
-	return free;
+	return max_t(long, free, 0);
 }
 
 /*