The patch below does not apply to the 5.4-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 stable@vger.kernel.org.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 5a3b590d4b2db187faa6f06adc9a53d6199fb1f9 Mon Sep 17 00:00:00 2001
From: Theodore Ts'o tytso@mit.edu Date: Thu, 17 Dec 2020 13:24:15 -0500 Subject: [PATCH] ext4: don't leak old mountpoint samples
When the first file is opened, ext4 samples the mountpoint of the filesystem in 64 bytes of the super block. It does so using strlcpy(), this means that the remaining bytes in the super block string buffer are untouched. If the mount point before had a longer path than the current one, it can be reconstructed.
Consider the case where the fs was mounted to "/media/johnjdeveloper" and later to "/". The super block buffer then contains "/\x00edia/johnjdeveloper".
This case was seen in the wild and caused confusion how the name of a developer ands up on the super block of a filesystem used in production...
Fix this by using strncpy() instead of strlcpy(). The superblock field is defined to be a fixed-size char array, and it is already marked using __nonstring in fs/ext4/ext4.h. The consumer of the field in e2fsprogs already assumes that in the case of a 64+ byte mount path, that s_last_mounted will not be NUL terminated.
Link: https://lore.kernel.org/r/X9ujIOJG/HqMr88R@mit.edu Reported-by: Richard Weinberger richard@nod.at Signed-off-by: Theodore Ts'o tytso@mit.edu Cc: stable@kernel.org
diff --git a/fs/ext4/file.c b/fs/ext4/file.c index 1cd3d26e3217..349b27f0dda0 100644 --- a/fs/ext4/file.c +++ b/fs/ext4/file.c @@ -810,7 +810,7 @@ static int ext4_sample_last_mounted(struct super_block *sb, if (err) goto out_journal; lock_buffer(sbi->s_sbh); - strlcpy(sbi->s_es->s_last_mounted, cp, + strncpy(sbi->s_es->s_last_mounted, cp, sizeof(sbi->s_es->s_last_mounted)); ext4_superblock_csum_set(sb); unlock_buffer(sbi->s_sbh);