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    SubjectRe: [SYSFS] Kernel Null pointer dereference in sysfs_readdir()
    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@goodmis.org>
    >
    > 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@in.ibm.com>
    ---

    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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