linux-kernel - v2.6.28-rc1: readlink /proc/*/exe returns uninitialized data to userspace

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Date:	Sat, 25 Oct 2008 19:14:21 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Al Viro" <viro@...iv.linux.org.uk>,
	"Alexey Dobriyan" <adobriyan@...il.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Pekka Enberg" <penberg@...helsinki.fi>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>
Subject: v2.6.28-rc1: readlink /proc/*/exe returns uninitialized data to userspace

Hi,

When I run readlink on the /proc/*/exe-file for udevd, the kernel
returns some unitialized data to userspace:

# strace -e trace=readlink readlink /proc/4762/exe
readlink("/proc/4762/exe", "/sbin/udevd", 1025) = 30

You can see it because the kernel thinks that the string is 30 bytes
long, but in fact it is only 12 (including the '\0').

If we explicitly clear the buffer before calling readlink, we can also
see that some garbage has been filled in there, after the string:

# ./readlink /proc/4762/exe
readlink(/proc/4762/exe) = 30
2f7362696e2f7564657664000000ffffffad4effffffadffffffdeffffffffffffffff202864656c657465642900000000000000000000000000000

(Output is from following simple program:)

#include <stdio.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
        char buf[1024];
        int i;
        ssize_t n;

        memset(buf, 0, sizeof(buf));
        n = readlink(argv[1], buf, sizeof(buf));

        printf("readlink(%s) = %d\n", argv[1], n);

        for (i = 0; i < sizeof(buf); ++i)
                printf("%02x", buf[i]);
        printf("\n");

        return 0;
}

It was discovered by kmemcheck:

WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (f6a109e4)
64000000ad4eaddeffffffffffffffff000000000200000000000000c0838ff8
 i i u u u u u u u u u u u u u u u u u u u u u u u u u u u u u u
         ^

Pid: 21511, comm: readlink Not tainted (2.6.28-rc1 #58) 945P-A
EIP: 0060:[<c04f988d>] EFLAGS: 00000296 CPU: 0
EIP is at __d_path+0x8d/0x1c0
EAX: 0000000e EBX: d7ba0fe7 ECX: 00000001 EDX: f68b0b40
ESI: f6a109e4 EDI: d7ba0fef EBP: e58c3f28 ESP: c2569c08
 DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
CR0: 8005003b CR2: f6c1d704 CR3: 31fc7000 CR4: 00000650
DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
DR6: ffff4ff0 DR7: 00000400
 [<c04fa4b0>] d_path+0xb0/0xd0
 [<c052c37c>] proc_pid_readlink+0x6c/0xc0
 [<c04eda34>] sys_readlinkat+0x94/0xa0
 [<c04eda67>] sys_readlink+0x27/0x30
 [<c0422f83>] sysenter_do_call+0x12/0x3f
 [<ffffffff>] 0xffffffff

Line numbers are these (as of commit
e013e13bf605b9e6b702adffbe2853cfc60e7806 in Linus's tree):

$ addr2line -e vmlinux -i c04f988d c04fa4b0 c052c37c c04eda34 c04eda67
fs/dcache.c:1895
fs/dcache.c:1901
fs/dcache.c:1957
fs/dcache.c:2016
fs/proc/base.c:1347
fs/proc/base.c:1374
fs/stat.c:312
fs/stat.c:325

I couldn't immediately figure out who/what to blame, please Cc in
right direction if you think you know it :-)

(For the record: This didn't show up in 2.6.27-rc with the same
version of LTP, so it seems to be a recent regression.)


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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