On Tue, 19 Feb 2013, Greg Thelen wrote: > The tmpfs remount logic preserves filesystem mempolicy if the mpol=M > option is not specified in the remount request. A new policy can be > specified if mpol=M is given. > > Before this patch remounting an mpol bound tmpfs without specifying > mpol= mount option in the remount request would set the filesystem's > mempolicy object to a freed mempolicy object. > > To reproduce the problem boot a DEBUG_PAGEALLOC kernel and run: > # mkdir /tmp/x > > # mount -t tmpfs -o size=100M,mpol=interleave nodev /tmp/x > > # grep /tmp/x /proc/mounts > nodev /tmp/x tmpfs rw,relatime,size=102400k,mpol=interleave:0-3 0 0 > > # mount -o remount,size=200M nodev /tmp/x > > # grep /tmp/x /proc/mounts > nodev /tmp/x tmpfs rw,relatime,size=204800k,mpol=??? 0 0 > # note ? garbage in mpol=... output above > > # dd if=/dev/zero of=/tmp/x/f count=1 > # panic here > > Panic: > BUG: unable to handle kernel NULL pointer dereference at (null) > IP: [< (null)>] (null) > [...] > Oops: 0010 [#1] SMP DEBUG_PAGEALLOC > Call Trace: > [<ffffffff81186ead>] ? mpol_set_nodemask+0x8d/0x100 > [<ffffffff811895ef>] ? mpol_shared_policy_init+0x8f/0x160 > [<ffffffff81189605>] mpol_shared_policy_init+0xa5/0x160 > [<ffffffff811580e1>] ? shmem_get_inode+0x1e1/0x270 > [<ffffffff811580e1>] ? shmem_get_inode+0x1e1/0x270 > [<ffffffff810db15d>] ? trace_hardirqs_on+0xd/0x10 > [<ffffffff81158109>] shmem_get_inode+0x209/0x270 > [<ffffffff811581ae>] shmem_mknod+0x3e/0xf0 > [<ffffffff811582b8>] shmem_create+0x18/0x20 > [<ffffffff811af5d5>] vfs_create+0xb5/0x130 > [<ffffffff811afff1>] do_last+0x9a1/0xea0 > [<ffffffff811ac77a>] ? link_path_walk+0x7a/0x930 > [<ffffffff811b05a3>] path_openat+0xb3/0x4d0 > [<ffffffff811be831>] ? __alloc_fd+0x31/0x160 > [<ffffffff811b0de2>] do_filp_open+0x42/0xa0 > [<ffffffff811be8e0>] ? __alloc_fd+0xe0/0x160 > [<ffffffff811a066e>] do_sys_open+0xfe/0x1e0 > [<ffffffff811f0aeb>] compat_sys_open+0x1b/0x20 > [<ffffffff815d6055>] cstar_dispatch+0x7/0x1f > > Non-debug kernels will not crash immediately because referencing the > dangling mpol will not cause a fault. Instead the filesystem will > reference a freed mempolicy object, which will cause unpredictable > behavior. > > The problem boils down to a dropped mpol reference below if > shmem_parse_options() does not allocate a new mpol: > config = *sbinfo > shmem_parse_options(data, &config, true) > mpol_put(sbinfo->mpol) > sbinfo->mpol = config.mpol /* BUG: saves unreferenced mpol */ > > This patch avoids the crash by not releasing the mempolicy if > shmem_parse_options() doesn't create a new mpol. This looks very good to me, thank you Greg. I did start to wonder about the consequence of an error in shmem_parse_options() after allocating mempolicy, but you're ahead of me, and have sensibly deferred leak fixes to 2/2. > > How far back does this issue go? I see it in both 2.6.36 and 3.3. I > did not look back further. It probably goes back to 2.6.26, when the refcounting came in at this mount level. But there's been such a sad history of bugs in mempolicy refcounting, occasionally one bug has protected against another. And whether anyone actually uses these tmpfs mpol= options is another question. 2.6.26 claimed to add support for "mpol=local", yet nobody noticed that it was rejected as an invalid option, until an independent bugfix in 2.6.34 rearranged the code and happened to get it working. I only discovered that a month or two ago, when working to fix the corruption which 2.6.35 then immediately bestowed upon "mpol=local". > > Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> I'm inclined to add Cc: stable@xxxxxxxxxxxxxxx to this one, but not to the 2/2 leak one; but I'm perfectly happy if Andrew chooses differently when he picks them up. > --- > mm/shmem.c | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/mm/shmem.c b/mm/shmem.c > index 5dd56f6..efd0b3a 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -2487,6 +2487,7 @@ static int shmem_remount_fs(struct super_block *sb, int *flags, char *data) > unsigned long inodes; > int error = -EINVAL; > > + config.mpol = NULL; > if (shmem_parse_options(data, &config, true)) > return error; > > @@ -2511,8 +2512,13 @@ static int shmem_remount_fs(struct super_block *sb, int *flags, char *data) > sbinfo->max_inodes = config.max_inodes; > sbinfo->free_inodes = config.max_inodes - inodes; > > - mpol_put(sbinfo->mpol); > - sbinfo->mpol = config.mpol; /* transfers initial ref */ > + /* > + * Preserve previous mempolicy unless mpol remount option was specified. > + */ > + if (config.mpol) { > + mpol_put(sbinfo->mpol); > + sbinfo->mpol = config.mpol; /* transfers initial ref */ > + } > out: > spin_unlock(&sbinfo->stat_lock); > return error; > -- > 1.8.1.3 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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