Messages in this thread Patch in this message | | | Subject | Re: [Cluster-devel] [PATCH] fs: gfs2: prevent OOB access in gfs2_read_sb() | From | Andrew Price <> | Date | Wed, 14 Oct 2020 14:04:10 +0100 |
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On 13/10/2020 16:26, Anant Thazhemadam wrote: > In gfs2_read_sb(), if the condition > (d != sdp->sd_heightsize[x - 1] || m) > isn't satisfied (in the first 11 iterations), the loop continues, > and begins to perform out-of-bounds access. > Fix this out-of-bounds access by introducing a condition in the for loop > that ensures that no more than GFS2_MAX_META_HEIGHT + 1 elements are > accessed. > > In addition to this, if the above condition is satisfied when > x = GFS2_MAX_META_HEIGHT (which = 10), and the flow of control breaks > out of the loop, then an out-of-bounds access is performed again while > assigning sdp->sd_heightsize[x] = ~0 (since x would be 11 now.), and > also the assertion that spd->sd_max_height <= GFS2_MAX_META_HEIGHT would > fail. > Fix this out-of-bounds access and ensure that the assertion doesn't fail > by introducing another variable "index", which keeps track of the last > valid value of x (pre-increment) that can be used.
That's not quite the right approach. Your analysis below is correct: the problem stems from the block size in the superblock being zeroed by the fuzzer. So the correct fix would be to add a validation check for sb_bsize (gfs2_check_sb() is lacking somewhat). Valid values are powers of 2 between 512 and the page size.
Just a heads-up to avoid duplication of effort: Fox Chen (CCed) has attempted to fix this also[1], but I don't know if they plan to send another patch.
[1] https://www.redhat.com/archives/cluster-devel/2020-October/msg00006.html
Thanks, Andy
> Reported-by: syzbot+a5e2482a693e6b1e444b@syzkaller.appspotmail.com > Tested-by: syzbot+a5e2482a693e6b1e444b@syzkaller.appspotmail.com > Signed-off-by: Anant Thazhemadam <anant.thazhemadam@gmail.com> > --- > > I have one question here (potentially a place where I suspect this > patch could have a fatal flaw and might need some rework). > > sdp->sd_max_height = x; > sdp->sd_heightsize[x] = ~0; > > Were these lines written with the logic that the value of x would be > equal to (sdp->sd_heightsize[]'s last index filled in by the loop) + 1? > Or, is the expected value of x at these lines equal to > (sdp->sd_heightsize[]'s last index as filled in by the loop)? > I would appreciate it if someone could clarify for me, how this would > hold against the second potential out-of-bounds access I mentioned in my > commit message. > > An additional comment (which I feel is of some significance) on this. > Reproducing the crash locally, I could infer that sdp->sd_fsb2bb_shift > sdp->sd_sb.sb_bsize, sdp->sd_sb.sb_bsize_shift, and sdp->sd_inptrs > were all 0. > This by extension also means that in gfs2_read_sb(), all the attributes > whose values were determined by performing some sort of calculation > involving any one of these variables all resulted in either 0 or a > negative value. > Simply doing the math will also show how this was also the primary reason > this OOB access occured in the first place. > However, I still feel that gfs2_read_sb() could do with this bit of checking, > since it fundamentally prevents OOB accesses from occuring in gfs2_read_sb() > in all scenarios. > Anyways, coming back to my initial point. Can having values like that be > considered unacceptable and as something that needs to be handled (at > gfs2_fill_super() maybe?) or is this non-anomalous behaviour and okay? > > fs/gfs2/ops_fstype.c | 15 ++++++++------- > 1 file changed, 8 insertions(+), 7 deletions(-) > > diff --git a/fs/gfs2/ops_fstype.c b/fs/gfs2/ops_fstype.c > index 6d18d2c91add..66ee8fb06ab9 100644 > --- a/fs/gfs2/ops_fstype.c > +++ b/fs/gfs2/ops_fstype.c > @@ -281,7 +281,7 @@ static int gfs2_read_sb(struct gfs2_sbd *sdp, int silent) > { > u32 hash_blocks, ind_blocks, leaf_blocks; > u32 tmp_blocks; > - unsigned int x; > + unsigned int x, index; > int error; > > error = gfs2_read_super(sdp, GFS2_SB_ADDR >> sdp->sd_fsb2bb_shift, silent); > @@ -329,20 +329,21 @@ static int gfs2_read_sb(struct gfs2_sbd *sdp, int silent) > sdp->sd_heightsize[0] = sdp->sd_sb.sb_bsize - > sizeof(struct gfs2_dinode); > sdp->sd_heightsize[1] = sdp->sd_sb.sb_bsize * sdp->sd_diptrs; > - for (x = 2;; x++) { > + for (x = 2; x <= GFS2_MAX_META_HEIGHT; x++) { > u64 space, d; > u32 m; > > - space = sdp->sd_heightsize[x - 1] * sdp->sd_inptrs; > + index = x; > + space = sdp->sd_heightsize[index - 1] * sdp->sd_inptrs; > d = space; > m = do_div(d, sdp->sd_inptrs); > > - if (d != sdp->sd_heightsize[x - 1] || m) > + if (d != sdp->sd_heightsize[index - 1] || m) > break; > - sdp->sd_heightsize[x] = space; > + sdp->sd_heightsize[index] = space; > } > - sdp->sd_max_height = x; > - sdp->sd_heightsize[x] = ~0; > + sdp->sd_max_height = index; > + sdp->sd_heightsize[index] = ~0; > gfs2_assert(sdp, sdp->sd_max_height <= GFS2_MAX_META_HEIGHT); > > sdp->sd_max_dents_per_leaf = (sdp->sd_sb.sb_bsize - >
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