[Xen-changelog] [qemu-xen stable-4.12] 9p: fix QEMU crash when renaming files

[patchbot@xxxxxxx]

commit 4469cab68e539899cbc2b1eae54dbc5373de561c
Author:     Greg Kurz <groug@xxxxxxxx>
AuthorDate: Fri Nov 23 13:28:03 2018 +0100
Commit:     Anthony PERARD <anthony.perard@xxxxxxxxxx>
CommitDate: Tue Apr 2 17:27:33 2019 +0100

    9p: fix QEMU crash when renaming files
    
    When using the 9P2000.u version of the protocol, the following shell
    command line in the guest can cause QEMU to crash:
    
        while true; do rm -rf aa; mkdir -p a/b & touch a/b/c & mv a aa; done
    
    With 9P2000.u, file renaming is handled by the WSTAT command. The
    v9fs_wstat() function calls v9fs_complete_rename(), which calls
    v9fs_fix_path() for every fid whose path is affected by the change.
    The involved calls to v9fs_path_copy() may race with any other access
    to the fid path performed by some worker thread, causing a crash like
    shown below:
    
    Thread 12 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
    0x0000555555a25da2 in local_open_nofollow (fs_ctx=0x555557d958b8, path=0x0,
     flags=65536, mode=0) at hw/9pfs/9p-local.c:59
    59          while (*path && fd != -1) {
    (gdb) bt
    #0  0x0000555555a25da2 in local_open_nofollow (fs_ctx=0x555557d958b8,
     path=0x0, flags=65536, mode=0) at hw/9pfs/9p-local.c:59
    #1  0x0000555555a25e0c in local_opendir_nofollow (fs_ctx=0x555557d958b8,
     path=0x0) at hw/9pfs/9p-local.c:92
    #2  0x0000555555a261b8 in local_lstat (fs_ctx=0x555557d958b8,
     fs_path=0x555556b56858, stbuf=0x7fff84830ef0) at hw/9pfs/9p-local.c:185
    #3  0x0000555555a2b367 in v9fs_co_lstat (pdu=0x555557d97498,
     path=0x555556b56858, stbuf=0x7fff84830ef0) at hw/9pfs/cofile.c:53
    #4  0x0000555555a1e9e2 in v9fs_stat (opaque=0x555557d97498)
     at hw/9pfs/9p.c:1083
    #5  0x0000555555e060a2 in coroutine_trampoline (i0=-669165424, i1=32767)
     at util/coroutine-ucontext.c:116
    #6  0x00007fffef4f5600 in __start_context () at /lib64/libc.so.6
    #7  0x0000000000000000 in  ()
    (gdb)
    
    The fix is to take the path write lock when calling v9fs_complete_rename(),
    like in v9fs_rename().
    
    Impact:  DoS triggered by unprivileged guest users.
    
    Fixes: CVE-2018-19489
    Cc: P J P <ppandit@xxxxxxxxxx>
    Reported-by: zhibin hu <noirfate@xxxxxxxxx>
    Reviewed-by: Prasad J Pandit <pjp@xxxxxxxxxxxxxxxxx>
    Signed-off-by: Greg Kurz <groug@xxxxxxxx>
    (cherry picked from commit 1d20398694a3b67a388d955b7a945ba4aa90a8a8)
---
 hw/9pfs/9p.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index 267a25533b..bdf7919abf 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -2855,6 +2855,7 @@ static void coroutine_fn v9fs_wstat(void *opaque)
     struct stat stbuf;
     V9fsFidState *fidp;
     V9fsPDU *pdu = opaque;
+    V9fsState *s = pdu->s;
 
     v9fs_stat_init(&v9stat);
     err = pdu_unmarshal(pdu, offset, "dwS", &fid, &unused, &v9stat);
@@ -2920,7 +2921,9 @@ static void coroutine_fn v9fs_wstat(void *opaque)
         }
     }
     if (v9stat.name.size != 0) {
+        v9fs_path_write_lock(s);
         err = v9fs_complete_rename(pdu, fidp, -1, &v9stat.name);
+        v9fs_path_unlock(s);
         if (err < 0) {
             goto out;
         }
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#stable-4.12

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