Messages in this thread Patch in this message | | | From | Anant Thazhemadam <> | Subject | [PATCH] fs: gfs2: prevent OOB access in gfs2_read_sb() | Date | Tue, 13 Oct 2020 20:56:48 +0530 |
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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.
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 - -- 2.25.1
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