Re: v2.6.28-rc1: readlink /proc/*/exe returns uninitialized data touserspace
From: Andrew Morton
Date: Tue Nov 11 2008 - 17:14:52 EST
On Sat, 8 Nov 2008 02:12:05 +0300
Alexey Dobriyan <adobriyan@xxxxxxxxx> wrote:
> On Fri, Nov 07, 2008 at 11:05:44AM -0800, Greg KH wrote:
> > On Thu, Nov 06, 2008 at 11:04:10AM +0100, Ingo Molnar wrote:
> > >
> > > * Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > > On Tue, Nov 04, 2008 at 10:39:19AM +0100, Vegard Nossum wrote:
> > > > > On Sun, Oct 26, 2008 at 10:08 PM, Vegard Nossum <vegard.nossum@xxxxxxxxx> wrote:
> > > > > # uname -a
> > > > > Linux localhost.localdomain 2.6.25.11-97.fc9.i686 #1 SMP Mon Jul 21
> > > > > 01:31:09 EDT 2008 i686 i686 i386 GNU/Linux
> > > > > # prelink -mRf /sbin/udevd
> > > > > # ./a.out /proc/564/exe
> > > > > warning: /proc/564/exe: got return value 38, expected 11
> > > > > 2f7362696e2f7564657664005f47387942426e5952446e566f306868202864656c6574656429
> > > > > /sbin/udevd _G8yBBnYRDnVo0hh (deleted)
> > > > >
> > > > > Yoshiya Koyama reports that the problem exists on RHEL 2.6.9-42.ELsmp too.
> > > > >
> > > > > I don't think it's exactly the same problem as originally reported,
> > > > > because I definitely wasn't using prelinking (the prelink binary
> > > > > wasn't even installed on the machine until today). But finding the
> > > > > root cause of this may solve both problems.
> > > >
> > > > switch_names() buggered in case of short names on both sides. That should
> > > > help:
> > > >
> > > > >From 2acda856910b774717e0290bbf948c7dee0f2e1a Mon Sep 17 00:00:00 2001
> > > > From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > > > Date: Mon, 3 Nov 2008 15:03:50 -0500
> > > > Subject: [PATCH] fix switch_names() breakage in short-to-short case
> > > >
> > > > We want ->name.len to match the resulting name on *both*
> > > > source and target
> > > >
> > > > Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > >
> > > please credit kmemcheck in the commit message and use an appropriate
> > > Reported-by line as well. Thanks,
> >
> > Did this fix ever get merged into Linus's tree?
>
> So far no.
I queued the below for 2.6.28 inclusion and tagged for -stable
backporting.
From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Vegard sayeth:
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
We want ->name.len to match the resulting name on *both* source and target.
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: "Vegard Nossum" <vegard.nossum@xxxxxxxxx>
Cc: "Rafael J. Wysocki" <rjw@xxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: "Rafael J. Wysocki" <rjw@xxxxxxx>
Cc: <stable@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/dcache.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff -puN fs/dcache.c~vfs-fix-switch_names-breakage-in-short-to-short-case fs/dcache.c
--- a/fs/dcache.c~vfs-fix-switch_names-breakage-in-short-to-short-case
+++ a/fs/dcache.c
@@ -1621,8 +1621,11 @@ static void switch_names(struct dentry *
*/
memcpy(dentry->d_iname, target->d_name.name,
target->d_name.len + 1);
+ dentry->d_name.len = target->d_name.len;
+ return;
}
}
+ do_switch(dentry->d_name.len, target->d_name.len);
}
/*
@@ -1682,7 +1685,6 @@ already_unhashed:
/* Switch the names.. */
switch_names(dentry, target);
- do_switch(dentry->d_name.len, target->d_name.len);
do_switch(dentry->d_name.hash, target->d_name.hash);
/* ... and switch the parents */
@@ -1792,7 +1794,6 @@ static void __d_materialise_dentry(struc
struct dentry *dparent, *aparent;
switch_names(dentry, anon);
- do_switch(dentry->d_name.len, anon->d_name.len);
do_switch(dentry->d_name.hash, anon->d_name.hash);
dparent = dentry->d_parent;
_
--
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