• Daniel Borkmann's avatar
    bpf: fix use after free in bpf_evict_inode · 1da6c4d9
    Daniel Borkmann authored
    syzkaller was able to generate the following UAF in bpf:
    
      BUG: KASAN: use-after-free in lookup_last fs/namei.c:2269 [inline]
      BUG: KASAN: use-after-free in path_lookupat.isra.43+0x9f8/0xc00 fs/namei.c:2318
      Read of size 1 at addr ffff8801c4865c47 by task syz-executor2/9423
    
      CPU: 0 PID: 9423 Comm: syz-executor2 Not tainted 4.20.0-rc1-next-20181109+
      #110
      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+0x244/0x39d lib/dump_stack.c:113
        print_address_description.cold.7+0x9/0x1ff mm/kasan/report.c:256
        kasan_report_error mm/kasan/report.c:354 [inline]
        kasan_report.cold.8+0x242/0x309 mm/kasan/report.c:412
        __asan_report_load1_noabort+0x14/0x20 mm/kasan/report.c:430
        lookup_last fs/namei.c:2269 [inline]
        path_lookupat.isra.43+0x9f8/0xc00 fs/namei.c:2318
        filename_lookup+0x26a/0x520 fs/namei.c:2348
        user_path_at_empty+0x40/0x50 fs/namei.c:2608
        user_path include/linux/namei.h:62 [inline]
        do_mount+0x180/0x1ff0 fs/namespace.c:2980
        ksys_mount+0x12d/0x140 fs/namespace.c:3258
        __do_sys_mount fs/namespace.c:3272 [inline]
        __se_sys_mount fs/namespace.c:3269 [inline]
        __x64_sys_mount+0xbe/0x150 fs/namespace.c:3269
        do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
        entry_SYSCALL_64_after_hwframe+0x49/0xbe
      RIP: 0033:0x457569
      Code: fd b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7
      48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
      ff 0f 83 cb b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
      RSP: 002b:00007fde6ed96c78 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
      RAX: ffffffffffffffda RBX: 0000000000000005 RCX: 0000000000457569
      RDX: 0000000020000040 RSI: 0000000020000000 RDI: 0000000000000000
      RBP: 000000000072bf00 R08: 0000000020000340 R09: 0000000000000000
      R10: 0000000000200000 R11: 0000000000000246 R12: 00007fde6ed976d4
      R13: 00000000004c2c24 R14: 00000000004d4990 R15: 00000000ffffffff
    
      Allocated by task 9424:
        save_stack+0x43/0xd0 mm/kasan/kasan.c:448
        set_track mm/kasan/kasan.c:460 [inline]
        kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:553
        __do_kmalloc mm/slab.c:3722 [inline]
        __kmalloc_track_caller+0x157/0x760 mm/slab.c:3737
        kstrdup+0x39/0x70 mm/util.c:49
        bpf_symlink+0x26/0x140 kernel/bpf/inode.c:356
        vfs_symlink+0x37a/0x5d0 fs/namei.c:4127
        do_symlinkat+0x242/0x2d0 fs/namei.c:4154
        __do_sys_symlink fs/namei.c:4173 [inline]
        __se_sys_symlink fs/namei.c:4171 [inline]
        __x64_sys_symlink+0x59/0x80 fs/namei.c:4171
        do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
        entry_SYSCALL_64_after_hwframe+0x49/0xbe
    
      Freed by task 9425:
        save_stack+0x43/0xd0 mm/kasan/kasan.c:448
        set_track mm/kasan/kasan.c:460 [inline]
        __kasan_slab_free+0x102/0x150 mm/kasan/kasan.c:521
        kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
        __cache_free mm/slab.c:3498 [inline]
        kfree+0xcf/0x230 mm/slab.c:3817
        bpf_evict_inode+0x11f/0x150 kernel/bpf/inode.c:565
        evict+0x4b9/0x980 fs/inode.c:558
        iput_final fs/inode.c:1550 [inline]
        iput+0x674/0xa90 fs/inode.c:1576
        do_unlinkat+0x733/0xa30 fs/namei.c:4069
        __do_sys_unlink fs/namei.c:4110 [inline]
        __se_sys_unlink fs/namei.c:4108 [inline]
        __x64_sys_unlink+0x42/0x50 fs/namei.c:4108
        do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
        entry_SYSCALL_64_after_hwframe+0x49/0xbe
    
    In this scenario path lookup under RCU is racing with the final
    unlink in case of symlinks. As Linus puts it in his analysis:
    
      [...] We actually RCU-delay the inode freeing itself, but
      when we do the final iput(), the "evict()" function is called
      synchronously. Now, the simple fix would seem to just RCU-delay
      the kfree() of the symlink data in bpf_evict_inode(). Maybe
      that's the right thing to do. [...]
    
    Al suggested to piggy-back on the ->destroy_inode() callback in
    order to implement RCU deferral there which can then kfree() the
    inode->i_link eventually right before putting inode back into
    inode cache. By reusing free_inode_nonrcu() from there we can
    avoid the need for our own inode cache and just reuse generic
    one as we currently do.
    
    And in-fact on top of all this we should just get rid of the
    bpf_evict_inode() entirely. This means truncate_inode_pages_final()
    and clear_inode() will then simply be called by the fs core via
    evict(). Dropping the reference should really only be done when
    inode is unhashed and nothing reachable anymore, so it's better
    also moved into the final ->destroy_inode() callback.
    
    Fixes: 0f98621b ("bpf, inode: add support for symlinks and fix mtime/ctime")
    Reported-by: syzbot+fb731ca573367b7f6564@syzkaller.appspotmail.com
    Reported-by: syzbot+a13e5ead792d6df37818@syzkaller.appspotmail.com
    Reported-by: syzbot+7a8ba368b47fdefca61e@syzkaller.appspotmail.com
    Suggested-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    Analyzed-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
    Acked-by: default avatarAlexei Starovoitov <ast@kernel.org>
    Acked-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Acked-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    Link: https://lore.kernel.org/lkml/0000000000006946d2057bbd0eef@google.com/T/
    1da6c4d9
inode.c 13.6 KB