Re: [SYSFS] Kernel Null pointer dereference in sysfs_readdir()

From: Maneesh Soni
Date: Wed Jul 12 2006 - 15:55:16 EST

On Wed, Jul 12, 2006 at 10:06:19AM -0400, Steven Rostedt wrote:
> [ added Patrick since his name is on the sysfs documentation ]
> [ added Andrew since I am including a patch for 2.6.18 ]
>
> On Wed, 2006-07-12 at 14:39 +0200, Duetsch, Thomas LDE1 wrote:
> > Steven Rostedt wrote:
> > > On Wed, 2006-07-12 at 13:35 +0200, Duetsch, Thomas LDE1 wrote:
> > >> Hi,
> > >>
> > >> I'm currently working on a custom kernel based on Ingo's -rt patch
> > >> (2.6.16-rt29).
> > >>
> > >> While rebooting my machine, I came across a kernel null pointer
> > >> dereference in this code segment in fs/sysfs/dir.c, function
> > >> sysfs_readdir():
> > >>
> > >> for (p=q->next; p!= &parent_sd->s_children; p=p->next) {
> > struct
> > >> sysfs_dirent *next; const char * name;
> > >> int len;
> > >>
> > >> next = list_entry(p, struct sysfs_dirent,
> > >> s_sibling);
> > >> if (!next->s_element)
> > >> continue;
> > >>
> > >> name = sysfs_get_name(next);
> > >> len = strlen(name);
> > >> if (next->s_dentry)
> > >> PROBLEM -> ino = next->s_dentry->d_inode->i_ino;
> > >> else
> > >> ino = iunique(sysfs_sb, 2);
> > >>
> > >
> > > Hi Thomas,
> > >
> > > Do you have a backtrace to look at? It might be helpful to see what
> > > functions brought us to this point. Also it might help to determine if
> > > the problem is vanilla, -rt, or the custom kernel.
> > >
> > > Thanks,
> > >
> > > -- Steve
> >
> > Of course, here you go:
> >
> > <1>BUG: Unable to handle kernel NULL pointer dereference at virtual
> > address 00000020
> > <1> printing eip:
> > <4>c01af187
> > <1>*pde = 00000000
> > <0>Oops: 0000 [#1]
> > <0>PREEMPT
> > <4>cpu=0 current find:2667
> > <4>current resource_set: 0
> > <4> [<c0103fb1>] show_trace+0x21/0x30 (20)
> > <4> [<c01040ee>] dump_stack+0x1e/0x30 (20)
> > <4> [<c01044ca>] die+0xba/0x1b0 (68)
> > <4> [<c01146ed>] do_page_fault+0x37d/0x6d0 (96)
> > <4> [<c0103b0b>] error_code+0x4f/0x54 (120)
> > <4> [<c0182a60>] vfs_readdir+0xa0/0xc0 (36)
> > <4> [<c0182e85>] sys_getdents64+0x75/0xd0 (64)
> > <4> [<c0102fd9>] syscall_call+0x7/0xb (-8116)
> > <4>Modules linked in: ide_dump diskdump ipv6 eeprom adm1021 hwmon
> > i2c_piix4 i2c_core NPCI pcnet32 e100 unix
> > <0>CPU: 0
> > <4>EIP: 0060:[<c01af187>] Tainted: P VLI
> > <4>EFLAGS: 00010286 (2.6.16-rt29-rs_1.0 #1)
> > <0>EIP is at sysfs_readdir+0xd7/0x240
> > <0>eax: 00000000 ebx: cfd095a0 ecx: 00000005 edx: cf02be88
> > <0>esi: cfd0959c edi: ce150c16 ebp: c35b5f50 esp: c35b5f14
> > <0>ds: 007b es: 007b ss: 0068 preempt: 00000001
> > <0>Process find (pid: 2667, threadinfo=c35b4000 task=c3686730
> > wchan=00000000) stack_left=7900 worst_left=-1)
> > <0>Stack: <0>cfd0959c c3618e60 00000004 00000003 00000000 00000d74
> > 00000004 cfe46a58
> > <0> 00000005 ce150c10 cf02bb68 cfe46a4c ca844f20 fffffffe cfe47e40
> > c35b5f74
> > <0> c0182a60 ca844f20 c35b5f98 c0182d20 cfe47ec8 00001000 ca844f20
> > c0182d20
> > <0>Call Trace:
> > <4>cpu=0 current find:2667
> > <4>current resource_set: 0
> > <0> [<c010406a>] show_stack_log_lvl+0xaa/0xe0 (32)
> > <0> [<c01042ca>] show_registers+0x1ca/0x250 (64)
> > <0> [<c0104508>] die+0xf8/0x1b0 (68)
> > <0> [<c01146ed>] do_page_fault+0x37d/0x6d0 (96)
> > <0> [<c0103b0b>] error_code+0x4f/0x54 (120)
> > <0> [<c0182a60>] vfs_readdir+0xa0/0xc0 (36)
> > <0> [<c0182e85>] sys_getdents64+0x75/0xd0 (64)
> > <0> [<c0102fd9>] syscall_call+0x7/0xb (-8116)
> > <0>Code: 8d 74 26 00 89 34 24 e8 68 e4 ff ff 89 45 e8 b9 ff ff ff ff 31
> > c0 8b 7d e8 f2 ae f7 d1 49 89 4d e4 8b 46 20 85 c0 74 74 8b 40 20 <8b>
> > 50 20
> > 0f b7 46 1c 89 54 24 14 8b 4d 08 c1 e8 0c 89 44 24 18
> >
> > I hope that helps,
> >
>
> I believe I found the race. Here's the problem:
>
> There's no real protection in the sysfs_readdir regarding that for loop.
> So the if statement
>
> if (next->s_dentry)
> ino = next->s_dentry->d_inode->i_ino;
>
> has the race.
>
> We _can_ have a s_dentry without a d_inode! Reason is that in attaching an
> attribute we have: in fs/sysfs/dir.c sysfs_attach_attr()
>
> sd->s_dentry = dentry;
> error = sysfs_create(dentry, (attr->mode & S_IALLUGO) | S_IFREG, init);
>
> Where the dentry->d_inode can be NULL. I don't see any protection in
> this code to prevent this from happening.
>

sysfs_attach_attr() is called from sysfs_lookup() only, and which in turn
is called under parent inode's i_mutex from VFS layer.

But you did help in spotting a bug which could happen like this

i_mutext held
sysfs_lookup()
-->sysfs_attach_attr()
--> sysfs_create() fails
--> sd->s_dentry has a NULL d_inode
--> sysfs_put() frees the sysfs_dirent
--> error returned to lookup
i_mutex released

But the sysfs_dirent with NULL d_inode is never unlinked from
the parent sysfs_dirent. And later on this happens

vfs_readdir()
i_mutex held
-->sysfs_readdir()
--> trips on the freed sysfs_dirent with NULL inode.

I am not sure if it is possible for other thread to see the freed
sysfs_dirent and trip at sd->s_dentry->d_inode but the sysfs_dirent
should have been unlinked from the parent sysfs_dirent's s_children list.

> Now the question is, is it safe to add the test for s_dentry->d_inode too.
> I ask this because the s_dentry is in the process of being filled, and
> I don't know what effect this will have on what readdir wants. I guess
> it may be safe, so I'm giving this patch:
>
> -- Steve
>
>
> Description:
>
> In the process of creating a sysfs attribute, we can have a state
> where the sysfs descriptor can have a dentry with a NULL inode.
>
> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
>
> Index: linux-2.6.18-rc1/fs/sysfs/dir.c
> ===================================================================
> --- linux-2.6.18-rc1.orig/fs/sysfs/dir.c 2006-07-12 09:43:10.000000000 -0400
> +++ linux-2.6.18-rc1/fs/sysfs/dir.c 2006-07-12 10:01:18.000000000 -0400
> @@ -445,7 +445,7 @@ static int sysfs_readdir(struct file * f
>
> name = sysfs_get_name(next);
> len = strlen(name);
> - if (next->s_dentry)
> + if (next->s_dentry && next->s_dentry->d_inode)
> ino = next->s_dentry->d_inode->i_ino;
> else
> ino = iunique(sysfs_sb, 2);
>

I think this patch only fixes the sympton. I have tried to keep the
assumption of no negative dentries (dentries with NULL d_inode) valid
in sysfs. So, this does indicate a bug.

Could you please try the following patch instead



o Unlink the half baked sysfs_dirent if sysfs_create() fails.

Signed-off-by: Maneesh Soni <maneesh@xxxxxxxxxx>
---

linux-2.6.18-rc1-git5-maneesh/fs/sysfs/dir.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletion(-)

diff -puN fs/sysfs/dir.c~fix-sysfs_create-errors fs/sysfs/dir.c
--- linux-2.6.18-rc1-git5/fs/sysfs/dir.c~fix-sysfs_create-errors 2006-07-13 01:21:25.000000000 +0530
+++ linux-2.6.18-rc1-git5-maneesh/fs/sysfs/dir.c 2006-07-13 01:21:25.000000000 +0530
@@ -212,6 +212,7 @@ static int sysfs_attach_attr(struct sysf
sd->s_dentry = dentry;
error = sysfs_create(dentry, (attr->mode & S_IALLUGO) | S_IFREG, init);
if (error) {
+ list_del_init(&sd->s_sibling);
sysfs_put(sd);
return error;
}
@@ -236,8 +237,10 @@ static int sysfs_attach_link(struct sysf
if (!err) {
dentry->d_op = &sysfs_dentry_ops;
d_rehash(dentry);
- } else
+ } else {
+ list_del_init(&sd->s_sibling);
sysfs_put(sd);
+ }

return err;
}
_

-
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