Marcin Szewczyk reported and diagnosed a bug in Debian's kernel packages that allows a denial of service (crash) by local users with access to an aufs filesystem. The bug is in a Debian-specific patch, not the upstream kernel or aufs code. The current version in Debian 7 'wheezy' (3.2.81-1) and the current proposed update to Debian 8 'jessie' (3.16.36-1 are affected. Ben. On Tue, 2016-08-30 at 22:33 +0200, Marcin Szewczyk wrote: > Hi, > > the wheezy kernel upgrade from 3.2.78-1 to 3.2.81-1 added the SETFL > fcntl support code (#627782) which unfortunately results in a kernel > Oops when the fcntl is called on a directory. This breaks e.g. copying > files from an AUFS filesystem on a remote machine using scp. > > Minimal code to reproduce the problem: > #v+ > #include <stdio.h> > #include <stdlib.h> > #include <fcntl.h> > > int main (int argc, char **argv) { > const char *fname = NULL; > int fd; > if (argc != 2) > exit (1); > fname = argv[1]; > fd = open (fname, O_RDONLY|O_NONBLOCK); > printf ("fd %d\n", fd); > fcntl (fd, F_SETFL, O_RDONLY); > return 0; > } > #v- > > Call the program on regular a file (nothing happens) and then on a > directory (Oops). > > The Oops happens in fs/fcntl.c: > #v+ > if (!error && filp->f_op->owner && > !strcmp(filp->f_op->owner->name, "aufs") && > strstr(filp->f_op->owner->version, "+setfl")) > error = filp->f_op->setfl(filp, arg); > #v- > > > > > From fs/aufs/inode.c: > #v+ > case S_IFREG: > [...] > inode->i_fop = &aufs_file_fop; > [...] > case S_IFDIR: > [...] > inode->i_fop = &aufs_dir_fop; > #v- > > The aufs_file_fop structure sets the value of the .setfl member to > aufs_setfl (f_op.c). aufs_dir_fop (dir.c) on the other hand does not. > > dmesg: > #v+ > [42990.915100] aufs 3.2.x+setfl-debian > [43046.383421] BUG: unable to handle kernel NULL pointer dereference > at (null) > [43046.384011] IP: [< (null)>] (null) > [43046.384369] PGD 3d0f1067 PUD 3b8cc067 PMD 0 > [43046.384688] Oops: 0010 [#1] SMP > [43046.385620] Call Trace: > [...] > [43046.385620] [<ffffffff81108701>] ? setfl+0xf1/0x157 > [43046.385620] [<ffffffff81108b9e>] ? sys_fcntl+0x1dc/0x3b0 > [43046.385620] [<ffffffff81358af2>] ? system_call_fastpath+0x16/0x1b > #v- > > gdb: > #v+ > 0xffffffff811086d3 <+195>: callq 0xffffffff811b2bd2 <strcmp> > 0xffffffff811086d8 <+200>: test %eax,%eax > 0xffffffff811086da <+202>: jne 0xffffffff81108705 <setfl+245> > 0xffffffff811086dc <+204>: mov 0xb0(%r13),%rdi > 0xffffffff811086e3 <+211>: mov $0xffffffff814dc9e4,%rsi > 0xffffffff811086ea <+218>: callq 0xffffffff811b2e25 <strstr> > 0xffffffff811086ef <+223>: test %rax,%rax > 0xffffffff811086f2 <+226>: je 0xffffffff81108705 <setfl+245> > 0xffffffff811086f4 <+228>: mov %rbp,%rsi > 0xffffffff811086f7 <+231>: mov %rbx,%rdi > 0xffffffff811086fa <+234>: callq *0xd0(%r14) > 0xffffffff81108701 <+241>: test %eax,%eax > #v- > > Naturally it happens both on i686 and amd64. > > BTW, changelog link on the package's page[1] is dead. > > Interesting changelog's part: > > * aufs: Make fcntl(F_SETFL, ...) work (Closes: #627782): > - for aufs: new f_op->setfl() to support fcntl(F_SETFL) > - aufs: implement new f_op->setfl() > - fs: Fix ABI change for aufs F_SETFL fix > > Is there any chance for a fix in some future wheezy-lts update? > > [1] https://packages.debian.org/wheezy/linux-image-3.2.0-4-amd64 > -- Ben Hutchings Anthony's Law of Force: Don't force it, get a larger hammer.
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