Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2019-12-30, Aleksa Sarai <cyphar@cyphar.com> wrote:
> On 2019-12-30, Al Viro <viro@zeniv.linux.org.uk> wrote:
> > On Mon, Dec 30, 2019 at 04:20:35PM +1100, Aleksa Sarai wrote:
> > 
> > > A reasonably detailed explanation of the issues is provided in the patch
> > > itself, but the full traces produced by both the oopses and deadlocks is
> > > included below (it makes little sense to include them in the commit since we
> > > are disabling this feature, not directly fixing the bugs themselves).
> > > 
> > > I've posted this as an RFC on whether this feature should be allowed at
> > > all (and if anyone knows of legitimate uses for it), or if we should
> > > work on fixing these other kernel bugs that it exposes.
> > 
> > Umm...  Are all of those traces
> > 	a) reproducible on mainline and
> 
> This was on viro/for-next, I'll retry it on v5.5-rc4.

The NULL deref oops is reproducible on v5.5-rc4. Strangely it seems
harder to reproduce than on viro/for-next (I kept reproducing it there
by accident), but I'll double-check if that really is the case.

The simplest reproducer is (using the attached programs and .config):

  ln -s . link
  sudo ./umount_symlink link

There's also a few other whacky behaviours where you get -ELOOP or
-EACCES in cases where you shouldn't -- which results in MNT_DETACH
failing and the mount being impossible to get rid of. A good example is

  sudo ./mount_to_symlink /proc/self/exe link
  sudo ./umount_symlink link # -EACCES

Or

  ln -s . link1
  ln -s . link2
  sudo ./mount_to_symlink link1 link2
  sudo ./umount_symlink link1 # -ELOOP
  sudo ./umount_symlink link2 # -ELOOP

But I am trying to find a reproducer for the "umount of a mount
triggering an Oops" issue.

On another note -- I guess this is considered a feature which should
"just work" and not a bug?

    BUG: kernel NULL pointer dereference, address: 0000000000000000
    #PF: supervisor instruction fetch in kernel mode
    #PF: error_code(0x0010) - not-present page
    PGD 80000003c6fca067 P4D 80000003c6fca067 PUD 3c6f42067 PMD 0
    Oops: 0010 [#1] SMP PTI
    CPU: 4 PID: 4486 Comm: umount_symlink Tainted: G            E     5.5.0-rc4-cyphar #126
    Hardware name: LENOVO 20KHCTO1WW/20KHCTO1WW, BIOS N23ET55W (1.30 ) 08/31/2018
    RIP: 0010:0x0
    Code: Bad RIP value.
    RSP: 0018:ffffb70b82963cc0 EFLAGS: 00010206
    RAX: 0000000000000000 RBX: ffff906d0cc3bb40 RCX: 0000000000000abc
    RDX: 0000000000000089 RSI: ffff906d74623cc0 RDI: ffff906d74475df0
    RBP: ffff906d74475df0 R08: ffffd70b7fb24c20 R09: ffff906d066a5000
    R10: 0000000000000000 R11: 8080807fffffffff R12: ffff906d74623cc0
    R13: 0000000000000089 R14: ffffb70b82963dc0 R15: 0000000000000080
    FS:  00007fbc2a8f0540(0000) GS:ffff906dcf500000(0000) knlGS:0000000000000000
    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: ffffffffffffffd6 CR3: 00000003c68f8001 CR4: 00000000003606e0
    Call Trace:
     __lookup_slow+0x94/0x160
     lookup_slow+0x36/0x50
     path_mountpoint+0x1be/0x360
     filename_mountpoint+0xa5/0x150
     ? __lookup_hash+0xa0/0xa0
     ksys_umount+0x78/0x490
     __x64_sys_umount+0x12/0x20
     do_syscall_64+0x64/0x240
     entry_SYSCALL_64_after_hwframe+0x49/0xbe
    RIP: 0033:0x7fbc2a8274e7
	Code: 09 0c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 31 f6 e9 09
		  00 00 00 66 0f 1f 84 00 00 00 00 00 b8 a6 00 00 00 0f 05 <48> 3d 01 f0
		  ff ff 73 01 c3 48 8b 0d 69 09 0c 00 f7 d8 64 89 01 48
    RSP: 002b:00007ffd1da9b3f8 EFLAGS: 00000202 ORIG_RAX: 00000000000000a6
    RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fbc2a8274e7
    RDX: 0000000000000000 RSI: 0000000000000002 RDI: 0000000001300310
    RBP: 00007ffd1da9b4c0 R08: 0000000000000000 R09: 000000000000000f
    R10: 00007fbc2a92f800 R11: 0000000000000202 R12: 0000000000401090
    R13: 00007ffd1da9b5a0 R14: 0000000000000000 R15: 0000000000000000
    Modules linked in: [snip]
    CR2: 0000000000000000
    ---[ end trace ae473813e34e641d ]---
    RIP: 0010:0x0
    Code: Bad RIP value.
    RSP: 0018:ffffb70b82963cc0 EFLAGS: 00010206
    RAX: 0000000000000000 RBX: ffff906d0cc3bb40 RCX: 0000000000000abc
    RDX: 0000000000000089 RSI: ffff906d74623cc0 RDI: ffff906d74475df0
    RBP: ffff906d74475df0 R08: ffffd70b7fb24c20 R09: ffff906d066a5000
    R10: 0000000000000000 R11: 8080807fffffffff R12: ffff906d74623cc0
    R13: 0000000000000089 R14: ffffb70b82963dc0 R15: 0000000000000080
    FS:  00007fbc2a8f0540(0000) GS:ffff906dcf500000(0000) knlGS:0000000000000000
    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: ffffffffffffffd6 CR3: 00000003c68f8001 CR4: 00000000003606e0

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

[-- Attachment #1.2: .config --]
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[-- Attachment #1.3: mount_to_symlink.c --]
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#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/mount.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>

#define bail(msg) \
	do { printf("mount_to_symlink: %s: %m\n", msg); exit(1); } while (0)

int is_symlink(const char *path)
{
	struct stat stat = {};
	if (lstat(path, &stat) < 0)
		bail("lstat(<path>)");
	return S_ISLNK(stat.st_mode);
}

int main(int argc, char **argv)
{
	struct stat stat = {};
	char *src, *dst, *src_fdpath, *dst_fdpath;
	int src_fd, dst_fd;

	if (argc != 3)
		bail("usage: mount_to_symlink <src> <dst>");

	src_fdpath = src = argv[1];
	dst_fdpath = dst = argv[2];

	if (is_symlink(src)) {
		// open source fd
		src_fd = open(src, O_PATH | O_CLOEXEC | O_NOFOLLOW);
		if (src_fd < 0)
			bail("open(<src>, O_PATH|O_NOFOLLOW)");
		// construct fd path
		asprintf(&src_fdpath, "/proc/self/fd/%d", src_fd);
	}

	if (is_symlink(dst)) {
		// open target fd
		dst_fd = open(dst, O_PATH | O_CLOEXEC | O_NOFOLLOW);
		if (dst_fd < 0)
			bail("open(<dst>, O_PATH|O_NOFOLLOW)");
		// construct fd path
		asprintf(&dst_fdpath, "/proc/self/fd/%d", dst_fd);
	}

	// try to mount
	mount(src_fdpath, dst_fdpath, "", MS_BIND, "");
	printf("mount(%s, %s, MS_BIND) = %m (%d)\n", src, dst, -errno);
	return 0;
}

[-- Attachment #1.4: umount_symlink.c --]
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#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/mount.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>

#define bail(msg) \
	do { printf("mount_to_symlink: %s: %m\n", msg); exit(1); } while (0)

int main(int argc, char **argv)
{
	struct stat stat = {};
	char *mnt, *mnt_fdpath;
	int mnt_fd;

	if (argc != 2)
		bail("need <mount> argument");

	mnt = argv[1];

	// open mountpoint fd
	mnt_fd = open(mnt, O_PATH | O_CLOEXEC | O_NOFOLLOW);
	if (mnt_fd < 0)
		bail("open(<dst>, O_PATH|O_NOFOLLOW)");

	// get fdpaths
	asprintf(&mnt_fdpath, "/proc/self/fd/%d", mnt_fd);

	// try to mount
	umount2(mnt_fdpath, MNT_DETACH);
	printf("umount2(%s, MNT_DETACH) = %m (%d)\n", mnt, -errno);
	return 0;
}

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