[PATCH 4.2.y-ckt 181/268] prctl: take mmap sem for writing to protect against others

From: Kamal Mostafa
Date: Wed Jan 27 2016 - 16:07:36 EST

4.2.8-ckt3 -stable review patch. If anyone has any objections, please let me know.


From: Mateusz Guzik <mguzik@xxxxxxxxxx>

commit ddf1d398e517e660207e2c807f76a90df543a217 upstream.

An unprivileged user can trigger an oops on a kernel with

proc_pid_cmdline_read takes mmap_sem for reading and obtains args + env
start/end values. These get sanity checked as follows:
BUG_ON(arg_start > arg_end);
BUG_ON(env_start > env_end);

These can be changed by prctl_set_mm. Turns out also takes the semaphore for
reading, effectively rendering it useless. This results in:

kernel BUG at fs/proc/base.c:240!
invalid opcode: 0000 [#1] SMP
Modules linked in: virtio_net
CPU: 0 PID: 925 Comm: a.out Not tainted 4.4.0-rc8-next-20160105dupa+ #71
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
task: ffff880077a68000 ti: ffff8800784d0000 task.ti: ffff8800784d0000
RIP: proc_pid_cmdline_read+0x520/0x530
RSP: 0018:ffff8800784d3db8 EFLAGS: 00010206
RAX: ffff880077c5b6b0 RBX: ffff8800784d3f18 RCX: 0000000000000000
RDX: 0000000000000002 RSI: 00007f78e8857000 RDI: 0000000000000246
RBP: ffff8800784d3e40 R08: 0000000000000008 R09: 0000000000000001
R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000000050
R13: 00007f78e8857800 R14: ffff88006fcef000 R15: ffff880077c5b600
FS: 00007f78e884a740(0000) GS:ffff88007b200000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00007f78e8361770 CR3: 00000000790a5000 CR4: 00000000000006f0
Call Trace:
Code: 4c 8b 7d a8 eb e9 48 8b 9d 78 ff ff ff 4c 8b 7d 90 48 8b 03 48 39 45 a8 0f 87 f0 fe ff ff e9 d1 fe ff ff 4c 8b 7d 90 eb c6 0f 0b <0f> 0b 0f 0b 66 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00
RIP proc_pid_cmdline_read+0x520/0x530
---[ end trace 97882617ae9c6818 ]---

Turns out there are instances where the code just reads aformentioned
values without locking whatsoever - namely environ_read and get_cmdline.

Interestingly these functions look quite resilient against bogus values,
but I don't believe this should be relied upon.

The first patch gets rid of the oops bug by grabbing mmap_sem for

The second patch is optional and puts locking around aformentioned
consumers for safety. Consumers of other fields don't seem to benefit
from similar treatment and are left untouched.

This patch (of 2):

The code was taking the semaphore for reading, which does not protect
against readers nor concurrent modifications.

The problem could cause a sanity checks to fail in procfs's cmdline
reader, resulting in an OOPS.

Note that some functions perform an unlocked read of various mm fields,
but they seem to be fine despite possible modificaton.

Signed-off-by: Mateusz Guzik <mguzik@xxxxxxxxxx>
Acked-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Jarod Wilson <jarod@xxxxxxxxxx>
Cc: Jan Stancek <jstancek@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Anshuman Khandual <anshuman.linux@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
kernel/sys.c | 20 ++++++++++----------
1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 259fda2..af8c0b55 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1854,11 +1854,13 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
user_auxv[AT_VECTOR_SIZE - 1] = AT_NULL;

- if (prctl_map.exe_fd != (u32)-1)
+ if (prctl_map.exe_fd != (u32)-1) {
error = prctl_set_mm_exe_file(mm, prctl_map.exe_fd);
- down_read(&mm->mmap_sem);
- if (error)
- goto out;
+ if (error)
+ return error;
+ }
+ down_write(&mm->mmap_sem);

* We don't validate if these members are pointing to
@@ -1895,10 +1897,8 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
if (prctl_map.auxv_size)
memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));

- error = 0;
- up_read(&mm->mmap_sem);
- return error;
+ up_write(&mm->mmap_sem);
+ return 0;

@@ -1964,7 +1964,7 @@ static int prctl_set_mm(int opt, unsigned long addr,

error = -EINVAL;

- down_read(&mm->mmap_sem);
+ down_write(&mm->mmap_sem);
vma = find_vma(mm, addr);

prctl_map.start_code = mm->start_code;
@@ -2057,7 +2057,7 @@ static int prctl_set_mm(int opt, unsigned long addr,

error = 0;
- up_read(&mm->mmap_sem);
+ up_write(&mm->mmap_sem);
return error;