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Message-ID: <Y66LkPumQjHC2U7d@sol.localdomain> Date: Thu, 29 Dec 2022 22:56:16 -0800 From: Eric Biggers <ebiggers@...nel.org> To: Tudor Ambarus <tudor.ambarus@...aro.org> Cc: tytso@....edu, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org, joneslee@...gle.com, syzbot+0827b4b52b5ebf65f219@...kaller.appspotmail.com, stable@...r.kernel.org Subject: Re: [PATCH v2] ext4: Fix possible use-after-free in ext4_find_extent On Fri, Dec 30, 2022 at 08:29:31AM +0200, Tudor Ambarus wrote: > syzbot reported a use-after-free Read in ext4_find_extent that is hit when > using a corrupted file system. The bug was reported on Android 5.15, but > using the same reproducer triggers the bug on v6.2-rc1 as well. > > Fix the use-after-free by checking the extent header magic. An alternative > would be to check the values of EXT4_{FIRST,LAST}_{EXTENT,INDEX} used in > ext4_ext_binsearch() and ext4_ext_binsearch_idx(), so that we make sure > that pointers returned by EXT4_{FIRST,LAST}_{EXTENT,INDEX} don't exceed the > bounds of the extent tree node. But this alternative will not squash > the bug for the cases where eh->eh_entries fit into eh->eh_max. We could > also try to check the sanity of the path, but costs more than checking just > the header magic, so stick to the header magic sanity check. > > Link: https://syzkaller.appspot.com/bug?id=be6e90ce70987950e6deb3bac8418344ca8b96cd > Reported-by: syzbot+0827b4b52b5ebf65f219@...kaller.appspotmail.com > Cc: stable@...r.kernel.org > Signed-off-by: Tudor Ambarus <tudor.ambarus@...aro.org> > --- > v2: drop wrong/uneeded le16_to_cpu() conversion for eh->eh_magic > > fs/ext4/extents.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c > index 9de1c9d1a13d..bedc8c098449 100644 > --- a/fs/ext4/extents.c > +++ b/fs/ext4/extents.c > @@ -894,6 +894,12 @@ ext4_find_extent(struct inode *inode, ext4_lblk_t block, > gfp_flags |= __GFP_NOFAIL; > > eh = ext_inode_hdr(inode); > + if (eh->eh_magic != EXT4_EXT_MAGIC) { > + EXT4_ERROR_INODE(inode, "Extent header has invalid magic."); > + ret = -EFSCORRUPTED; > + goto err; > + } > + This is (incompletely) validating the extent header in the inode. Isn't that supposed to happen when the inode is loaded? See how __ext4_iget() calls ext4_ext_check_inode(). Why isn't that working here? - Eric
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