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Re: CVE-2011-4325 Linux kernel: nfs: diotest4 from LTP crash client
From: Petr Matousek <pmatouse () redhat com>
Date: Tue, 7 Feb 2012 20:23:09 +0100

Hi,

On Mon, Feb 06, 2012 at 01:42:48PM +0400, Solar Designer wrote:
<snip>
Apparently, an uninitialized pointer was being accessed, and apparently
it happened to be NULL (or nearby) on some occasion - but I see no proof
that it would always be NULL, although there may well be something that
makes it so.

Overall, after a quick glance at the fix, I am not convinced that this
was just a DoS.  Someone familiar with the code might have a better idea.

Code: 48 8b 47 38 48 89 fb 48 8b 68 10 48 8d b5 b4 00 00 00 e8 c9
RIP  [<ffffffff887aed65>] :nfs:__put_nfs_open_context+0x7/0x93
 RSP <ffff810153cc1d28>
CR2: 0000000000000038
 <0>Kernel panic - not syncing: Fatal exception

addr2line -ife nfs.ko.debug <<< 6D65
__put_nfs_open_context
/usr/src/debug/kernel-2.6.18/linux-2.6.18-296.el5.x86_64/fs/nfs/inode.c:624

0000000000006d5e <__put_nfs_open_context>:
    6d5e:       41 54                   push   %r12
    6d60:       41 89 f4                mov    %esi,%r12d
    6d63:       55                      push   %rbp
    6d64:       53                      push   %rbx
    6d65:       48 8b 47 38             mov    0x38(%rdi),%rax

static void __put_nfs_open_context(struct nfs_open_context *ctx, int is_sync)
{
        struct inode *inode = ctx->path.dentry->d_inode;   // line 624 / 6d65
        ...
}

[pmatouse () dhcp-25-225 linux-2.6.18-296.el5.x86_64]$ pahole -r -C nfs_open_context nfs.ko.debug
struct nfs_open_context {
        struct nfs_lock_context    lock_context;         /*     0    48 */
        struct path                path;                 /*    48    16 */
        ...
}

[pmatouse () dhcp-25-225 linux-2.6.18-296.el5.x86_64]$ pahole -r -C path nfs.ko.debug
struct path {
        struct vfsmount *          mnt;                  /*     0     8 */
        struct dentry *            dentry;               /*     8     8 */
        ...
}

48 + 8 == 56 (0x38) (mov    0x38(%rdi),%rax)
ctx is null

--

nfs_direct_read_schedule()
 -> data = nfs_readdata_alloc();                        // allocates and nulls readdata
 -> nfs_readdata_release(data);                         // on error
 -> data->args.context = get_nfs_open_context(ctx);     // initializes args.context

it can happen that nfs_readdata_release() is called with data->args.context
being NULL.

void nfs_readdata_release(void *data)
{
        struct nfs_read_data *rdata = data;

        put_nfs_open_context(rdata->args.context);
        nfs_readdata_free(rdata);
}

void put_nfs_open_context(struct nfs_open_context *ctx)
{
        __put_nfs_open_context(ctx, 0);
}

static void __put_nfs_open_context(struct nfs_open_context *ctx, int is_sync)
{
        struct inode *inode = ctx->path.dentry->d_inode;               // line 624
        ...
}

 
Also, does Red Hat treat NULL pointer derefs in the kernel as DoS only
now, relying primarily on mmap_min_addr to work?  (We do.  And we'll
treat a mmap_min_addr bypass if another one of these is found, as the
real privilege escalation issue, assuming that plenty of NULL derefs
exist in the kernel.)

Yes.

Thanks,
-- 
Petr Matousek / Red Hat Security Response Team


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