[PATCH 2/2] fsnotify: use method copy_dname copying filename

[石祤 <linxiulei@gmail.com>]

From: "leilei.lin" <leilei.lin@alibaba-inc.com>

Kernel got panicked in inotify_handle_event, while running the rename
operation against the same file. The root cause is that the race between
fsnotify thread and file rename thread.  When fsnotify thread was
copying the dentry name, another rename thread could change the dentry
name at same time. With slub_debug=FZ boot args, this bug will trigger
a trace like the following:

[   87.733653] =============================================================================
[   87.735350] BUG kmalloc-64 (Not tainted): Redzone overwritten
[   87.736550] -----------------------------------------------------------------------------

[   87.738466] Disabling lock debugging due to kernel taint
[   87.739556] INFO: 0xffff8e487a50b0f8-0xffff8e487a50b0fc. First byte 0x33 instead of 0xcc
[   87.741188] INFO: Slab 0xfffff116c0e942c0 objects=46 used=43 fp=0xffff8e487a50bf80 flags=0xffff8000000101
[   87.743133] INFO: Object 0xffff8e487a50b0b8 @offset=184 fp=0xffff8e487a50b0b8

[   87.744942] Redzone ffff8e487a50b0b0: cc cc cc cc cc cc cc cc                          ........
[   87.746743] Object ffff8e487a50b0b8: b8 b0 50 7a 48 8e ff ff b8 b0 50 7a 48 8e ff ff  ..PzH.....PzH...
[   87.748621] Object ffff8e487a50b0c8: 60 75 7e 7b 48 8e ff ff 08 00 00 08 00 00 00 00  `u~{H...........
[   87.750583] Object ffff8e487a50b0d8: 01 00 00 00 00 00 00 00 0d 00 00 00 74 64 63 5f  ............tdc_
[   87.752541] Object ffff8e487a50b0e8: 61 64 6d 69 6e 2e 4c 4f 47 2e 31 31 32 33 31 32  admin.LOG.112312
[   87.754431] Redzone ffff8e487a50b0f8: 33 31 32 33 00 cc cc cc                          3123....
[   87.756172] Padding ffff8e487a50b100: 00 00 00 00 00 00 00 00                          ........
[   87.757988] CPU: 0 PID: 286 Comm: python Tainted: G    B           4.11.0-rc4+ #29
[   87.759574] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.1-0-gb3ef39f-prebuilt.qemu-project.org 04/01/2014
[   87.761878] Call Trace:
[   87.762381]  dump_stack+0x65/0x88
[   87.763063]  print_trailer+0x15d/0x250
[   87.763833]  check_bytes_and_report+0xcd/0x110
[   87.764731]  check_object+0x1ce/0x290
[   87.765472]  free_debug_processing+0x9c/0x2e3
[   87.766362]  ? inotify_free_event+0xe/0x10
[   87.767191]  __slab_free+0x1ba/0x2b0
[   87.767922]  ? async_page_fault+0x28/0x30
[   87.768731]  ? inotify_free_event+0xe/0x10
[   87.769558]  kfree+0x165/0x1a0
[   87.770184]  inotify_free_event+0xe/0x10
[   87.770974]  fsnotify_destroy_event+0x51/0x70
[   87.771851]  inotify_read+0x1dc/0x3a0
[   87.772582]  ? do_wait_intr_irq+0xa0/0xa0
[   87.773388]  __vfs_read+0x37/0x150
[   87.774078]  ? security_file_permission+0x9d/0xc0
[   87.775009]  vfs_read+0x8c/0x130
[   87.775657]  SyS_read+0x55/0xc0
[   87.776328]  entry_SYSCALL_64_fastpath+0x1e/0xad
[   87.777280] RIP: 0033:0x7fcc1375b210
[   87.778001] RSP: 002b:00007ffe2f00b838 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[   87.779513] RAX: ffffffffffffffda RBX: 00007fcc1303d7b8 RCX: 00007fcc1375b210
[   87.780932] RDX: 0000000000005c70 RSI: 00000000013fe9f4 RDI: 0000000000000004
[   87.782337] RBP: 00007fcc1303d760 R08: 0000000000000080 R09: 0000000000005c95
[   87.783780] R10: 0000000000000073 R11: 0000000000000246 R12: 0000000000005c95
[   87.785203] R13: 0000000000002708 R14: 0000000000005ca1 R15: 00007fcc1303d7b8
[   87.786636] FIX kmalloc-64: Restoring 0xffff8e487a50b0f8-0xffff8e487a50b0fc=0xcc

[   87.789388] FIX kmalloc-64: Object at 0xffff8e487a50b0b8 not freed

Graph below is the flow of inotify subsystem handling
notify event. If a rename syscall happened simultaneously,
for example filename of "foobar" is rename to
"foobar_longername", which would access memory illegally.

            CPU 1                                       CPU 2

     fsnotify()
       inotify_handle_event()
         strlen(file_name) // file_name -> "foobar"

                                                    rename happen
                                                    file_name -> "foobar_longername"

         alloc_len += len + 1;
         event = kmalloc(alloc_len, GFP_KERNEL);
         strcpy(event->name, file_name); -> overwritten

Signed-off-by: leilei.lin <leilei.lin@alibaba-inc.com>
---
 fs/notify/fsnotify.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index b41515d..2c6f94d 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -91,6 +91,7 @@ int __fsnotify_parent(const struct path *path, struct dentry *dentry, __u32 mask
 	struct dentry *parent;
 	struct inode *p_inode;
 	int ret = 0;
+	char *name = NULL;
 
 	if (!dentry)
 		dentry = path->dentry;
@@ -108,14 +109,23 @@ int __fsnotify_parent(const struct path *path, struct dentry *dentry, __u32 mask
 		 * specifies these are events which came from a child. */
 		mask |= FS_EVENT_ON_CHILD;
 
+		ret = copy_dname(dentry, &name);
+
+		if (ret) {
+			dput(parent);
+			return ret;
+		}
+
 		if (path)
 			ret = fsnotify(p_inode, mask, path, FSNOTIFY_EVENT_PATH,
-				       dentry->d_name.name, 0);
+				       name, 0);
 		else
 			ret = fsnotify(p_inode, mask, dentry->d_inode, FSNOTIFY_EVENT_INODE,
-				       dentry->d_name.name, 0);
+				       name, 0);
 	}
 
+	kfree(name);
+
 	dput(parent);
 
 	return ret;
-- 
2.8.4.31.g9ed660f