From: Lior Ribak <liorribak@xxxxxxxxx> Subject: binfmt_misc: fix possible deadlock in bm_register_write There is a deadlock in bm_register_write: First, in the begining of the function, a lock is taken on the binfmt_misc root inode with inode_lock(d_inode(root)). Then, if the user used the MISC_FMT_OPEN_FILE flag, the function will call open_exec on the user-provided interpreter. open_exec will call a path lookup, and if the path lookup process includes the root of binfmt_misc, it will try to take a shared lock on its inode again, but it is already locked, and the code will get stuck in a deadlock To reproduce the bug: $ echo ":iiiii:E::ii::/proc/sys/fs/binfmt_misc/bla:F" > /proc/sys/fs/binfmt_misc/register backtrace of where the lock occurs (#5): 0 schedule () at ./arch/x86/include/asm/current.h:15 1 0xffffffff81b51237 in rwsem_down_read_slowpath (sem=0xffff888003b202e0, count=<optimized out>, state=state@entry=2) at kernel/locking/rwsem.c:992 2 0xffffffff81b5150a in __down_read_common (state=2, sem=<optimized out>) at kernel/locking/rwsem.c:1213 3 __down_read (sem=<optimized out>) at kernel/locking/rwsem.c:1222 4 down_read (sem=<optimized out>) at kernel/locking/rwsem.c:1355 5 0xffffffff811ee22a in inode_lock_shared (inode=<optimized out>) at ./include/linux/fs.h:783 6 open_last_lookups (op=0xffffc9000022fe34, file=0xffff888004098600, nd=0xffffc9000022fd10) at fs/namei.c:3177 7 path_openat (nd=nd@entry=0xffffc9000022fd10, op=op@entry=0xffffc9000022fe34, flags=flags@entry=65) at fs/namei.c:3366 8 0xffffffff811efe1c in do_filp_open (dfd=<optimized out>, pathname=pathname@entry=0xffff8880031b9000, op=op@entry=0xffffc9000022fe34) at fs/namei.c:3396 9 0xffffffff811e493f in do_open_execat (fd=fd@entry=-100, name=name@entry=0xffff8880031b9000, flags=<optimized out>, flags@entry=0) at fs/exec.c:913 10 0xffffffff811e4a92 in open_exec (name=<optimized out>) at fs/exec.c:948 11 0xffffffff8124aa84 in bm_register_write (file=<optimized out>, buffer=<optimized out>, count=19, ppos=<optimized out>) at fs/binfmt_misc.c:682 12 0xffffffff811decd2 in vfs_write (file=file@entry=0xffff888004098500, buf=buf@entry=0xa758d0 ":iiiii:E::ii::i:CF ", count=count@entry=19, pos=pos@entry=0xffffc9000022ff10) at fs/read_write.c:603 13 0xffffffff811defda in ksys_write (fd=<optimized out>, buf=0xa758d0 ":iiiii:E::ii::i:CF ", count=19) at fs/read_write.c:658 14 0xffffffff81b49813 in do_syscall_64 (nr=<optimized out>, regs=0xffffc9000022ff58) at arch/x86/entry/common.c:46 15 0xffffffff81c0007c in entry_SYSCALL_64 () at arch/x86/entry/entry_64.S:120 To solve the issue, the open_exec call is moved to before the write lock is taken by bm_register_write Link: https://lkml.kernel.org/r/20210228224414.95962-1-liorribak@xxxxxxxxx Fixes: 948b701a607f1 ("binfmt_misc: add persistent opened binary handler for containers") Signed-off-by: Lior Ribak <liorribak@xxxxxxxxx> Acked-by: Helge Deller <deller@xxxxxx> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/binfmt_misc.c | 29 ++++++++++++++--------------- 1 file changed, 14 insertions(+), 15 deletions(-) --- a/fs/binfmt_misc.c~binfmt_misc-fix-possible-deadlock-in-bm_register_write +++ a/fs/binfmt_misc.c @@ -649,12 +649,24 @@ static ssize_t bm_register_write(struct struct super_block *sb = file_inode(file)->i_sb; struct dentry *root = sb->s_root, *dentry; int err = 0; + struct file *f = NULL; e = create_entry(buffer, count); if (IS_ERR(e)) return PTR_ERR(e); + if (e->flags & MISC_FMT_OPEN_FILE) { + f = open_exec(e->interpreter); + if (IS_ERR(f)) { + pr_notice("register: failed to install interpreter file %s\n", + e->interpreter); + kfree(e); + return PTR_ERR(f); + } + e->interp_file = f; + } + inode_lock(d_inode(root)); dentry = lookup_one_len(e->name, root, strlen(e->name)); err = PTR_ERR(dentry); @@ -678,21 +690,6 @@ static ssize_t bm_register_write(struct goto out2; } - if (e->flags & MISC_FMT_OPEN_FILE) { - struct file *f; - - f = open_exec(e->interpreter); - if (IS_ERR(f)) { - err = PTR_ERR(f); - pr_notice("register: failed to install interpreter file %s\n", e->interpreter); - simple_release_fs(&bm_mnt, &entry_count); - iput(inode); - inode = NULL; - goto out2; - } - e->interp_file = f; - } - e->dentry = dget(dentry); inode->i_private = e; inode->i_fop = &bm_entry_operations; @@ -709,6 +706,8 @@ out: inode_unlock(d_inode(root)); if (err) { + if (f) + filp_close(f, NULL); kfree(e); return err; } _
