[ Upstream commit 1da6c4d9140cb7c13e87667dc4e1488d6c8fc10f ]
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: 0f98621bef5d ("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: Al Viro viro@zeniv.linux.org.uk Analyzed-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Daniel Borkmann daniel@iogearbox.net Acked-by: Alexei Starovoitov ast@kernel.org Acked-by: Linus Torvalds torvalds@linux-foundation.org Acked-by: Al Viro viro@zeniv.linux.org.uk Link: https://lore.kernel.org/lkml/0000000000006946d2057bbd0eef@google.com/T/ Signed-off-by: Sasha Levin (Microsoft) sashal@kernel.org --- kernel/bpf/inode.c | 32 ++++++++++++++++++-------------- 1 file changed, 18 insertions(+), 14 deletions(-)
diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c index be1dde967208..ccf9ffd5da78 100644 --- a/kernel/bpf/inode.c +++ b/kernel/bpf/inode.c @@ -365,19 +365,6 @@ int bpf_obj_get_user(const char __user *pathname) } EXPORT_SYMBOL_GPL(bpf_obj_get_user);
-static void bpf_evict_inode(struct inode *inode) -{ - enum bpf_type type; - - truncate_inode_pages_final(&inode->i_data); - clear_inode(inode); - - if (S_ISLNK(inode->i_mode)) - kfree(inode->i_link); - if (!bpf_inode_type(inode, &type)) - bpf_any_put(inode->i_private, type); -} - /* * Display the mount options in /proc/mounts. */ @@ -390,11 +377,28 @@ static int bpf_show_options(struct seq_file *m, struct dentry *root) return 0; }
+static void bpf_destroy_inode_deferred(struct rcu_head *head) +{ + struct inode *inode = container_of(head, struct inode, i_rcu); + enum bpf_type type; + + if (S_ISLNK(inode->i_mode)) + kfree(inode->i_link); + if (!bpf_inode_type(inode, &type)) + bpf_any_put(inode->i_private, type); + free_inode_nonrcu(inode); +} + +static void bpf_destroy_inode(struct inode *inode) +{ + call_rcu(&inode->i_rcu, bpf_destroy_inode_deferred); +} + static const struct super_operations bpf_super_ops = { .statfs = simple_statfs, .drop_inode = generic_delete_inode, .show_options = bpf_show_options, - .evict_inode = bpf_evict_inode, + .destroy_inode = bpf_destroy_inode, };
enum {