On Sat, May 23, 2020 at 1:23 AM syzbot <syzbot+61958888b1c60361a791@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > Hello, > > syzbot found the following crash on: > > HEAD commit: b85051e7 Merge tag 'fixes-for-5.7-rc6' of git://git.kernel.. > git tree: upstream > console output: https://syzkaller.appspot.com/x/log.txt?x=165d2b81100000 > kernel config: https://syzkaller.appspot.com/x/.config?x=b3368ce0cc5f5ace > dashboard link: https://syzkaller.appspot.com/bug?extid=61958888b1c60361a791 > compiler: gcc (GCC) 9.0.0 20181231 (experimental) > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=168e6272100000 > > The bug was bisected to: > > commit cbe7fba8edfc8cb8e621599e376f8ac5c224fa72 > Author: Amir Goldstein <amir73il@xxxxxxxxx> > Date: Fri Nov 15 11:33:03 2019 +0000 > > ovl: make sure that real fid is 32bit aligned in memory > > bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=11f95922100000 > final crash: https://syzkaller.appspot.com/x/report.txt?x=13f95922100000 > console output: https://syzkaller.appspot.com/x/log.txt?x=15f95922100000 > > IMPORTANT: if you fix the bug, please add the following tag to the commit: > Reported-by: syzbot+61958888b1c60361a791@xxxxxxxxxxxxxxxxxxxxxxxxx > Fixes: cbe7fba8edfc ("ovl: make sure that real fid is 32bit aligned in memory") > > ================================================================== > BUG: KASAN: slab-out-of-bounds in ovl_check_fb_len+0x171/0x1a0 fs/overlayfs/namei.c:89 > Read of size 1 at addr ffff88809727834d by task syz-executor.4/8488 > > CPU: 0 PID: 8488 Comm: syz-executor.4 Not tainted 5.7.0-rc6-syzkaller #0 > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 > Call Trace: > __dump_stack lib/dump_stack.c:77 [inline] > dump_stack+0x188/0x20d lib/dump_stack.c:118 > print_address_description.constprop.0.cold+0xd3/0x413 mm/kasan/report.c:382 > __kasan_report.cold+0x20/0x38 mm/kasan/report.c:511 > kasan_report+0x33/0x50 mm/kasan/common.c:625 > ovl_check_fb_len+0x171/0x1a0 fs/overlayfs/namei.c:89 > ovl_check_fh_len fs/overlayfs/overlayfs.h:358 [inline] > ovl_fh_to_dentry+0x1ab/0x814 fs/overlayfs/export.c:812 > exportfs_decode_fh+0x11f/0x717 fs/exportfs/expfs.c:434 > > repro crafts a file handle { .handle_bytes = 2, .handle_type = OVL_FILEID_V1 } handle_bytes gets rounded to 0, so we call ovl_check_fh_len(f_handle, 0) => ovl_check_fb_len(f_handle + 3, -3) I guess compiler may be evaluating the 2nd condition before the first: if (fb_len < sizeof(struct ovl_fb) || fb_len < fb->len) Silly thing is that Dan's patch that was just merged fixes a crash with: { .handle_bytes = 2, .handle_type = OVL_FILEID_V0 } The original patch that he sent would have caught this case as well, but I gave it a bad review comment, because I was too confident about ovl_check_fh_len()'s safety. But now I see that we also need to fix: { .handle_bytes = 4, .handle_type = OVL_FILEID_V0 } which wasn't covered even with the original fix patch. Let's try this fix: #syz test: https://github.com/amir73il/linux.git ovl-fixes Thanks, Amir.
