On Fri, 24 Sept 2021 at 03:09, Zheng Liang <zhengliang6@xxxxxxxxxx> wrote: > > We find a kernel NULL pointer dereference problem in overlayfs. > The problem can appear in the following scene: > > mkdir lower upper work merge > touch lower/old > touch lower/new > mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work merge > rm merge/new > ------------------------------------------------------------------------------------------ > process A(rename file in merge) process B(delete file in upper) > renameat2(AT_FDCWD, "old", AT_FDCWD, "new", 0) > (new is whiteout file in upper) > do_renameat2 > vfs_rename > ovl_rename > (overwrite=true,ovl_lower_positive(old)=true, > ovl_dentry_is_whiteout(new)=true) > (we can add some delay after "flags|=RENAME_EXCHANGE", > it can make the problem appear more easy) > …… unlink(new) > …… (delete whiteout in upper) > (newdentry is negative) > ovl_do_rename > > So,before commencing with ovl_do_rename that the flags maybe attach RENAME_EXCHANGE > and the newdentry is negative in ovl_rename.If we enabled selinux,it > will lead to kernel panic.such as the following log: > PID: 2552045 TASK: ffff8880302faf00 CPU: 2 COMMAND: "fsstress" > #0 [ffff888080e772a0] machine_kexec at ffffffff856adedc > #1 [ffff888080e773a8] __crash_kexec at ffffffff8585cd20 > #2 [ffff888080e774c0] panic at ffffffff8572b288 > #3 [ffff888080e77590] oops_end at ffffffff85641f6e > #4 [ffff888080e775f0] __do_page_fault at ffffffff856cd55b > #5 [ffff888080e77668] do_page_fault at ffffffff856cd834 > #6 [ffff888080e776a0] async_page_fault at ffffffff8660125e > [exception RIP: __inode_security_revalidate+34] > RIP: ffffffff85c43452 RSP: ffff888080e77758 RFLAGS: 00010202 > RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff8593ae7e > RDX: 0000000000000000 RSI: 0000000000000297 RDI: 0000000000000297 > RBP: ffff8881984e6628 R8: ffffed10115e3f39 R9: ffffed10115e3f39 > R10: 0000000000000001 R11: ffffed10115e3f38 R12: 0000000000000001 > R13: 0000000000000000 R14: ffff88808350a000 R15: 1ffff110101ceef5 > ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018 > #7 [ffff888080e77780] selinux_inode_rename at ffffffff85c4418d > #8 [ffff888080e77858] security_inode_rename at ffffffff85c35f24 > #9 [ffff888080e77890] vfs_rename at ffffffff85b01209 > #10 [ffff888080e779a8] ovl_do_rename at ffffffffc0a44c22 [overlay] > #11 [ffff888080e779d8] ovl_rename at ffffffffc0a46575 [overlay] > #12 [ffff888080e77b48] vfs_rename at ffffffff85b0155a > #13 [ffff888080e77c60] do_renameat2 at ffffffff85b06e65 > #14 [ffff888080e77f00] __x64_sys_renameat2 at ffffffff85b06fb2 > #15 [ffff888080e77f30] do_syscall_64 at ffffffff85606243 > #16 [ffff888080e77f50] entry_SYSCALL_64_after_hwframe at ffffffff866000ad > > We can add some check in ovl_rename for this scene and return error to avoid kernel panic. Thanks. Patch looks good; pushed to vfs.git#overlayfs-next (with a cleaned up commit message). Miklos
