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    SubjectRe: [Cluster-devel] [PATCH] fs: gfs2: prevent OOB access in gfs2_read_sb()
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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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