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Message-ID: <20200801150919.GA229082@PWN>
Date: Sat, 1 Aug 2020 11:09:19 -0400
From: Peilin Ye <yepeilin.cs@...il.com>
To: "Dmitry V. Levin" <ldv@...linux.org>
Cc: Oleg Nesterov <oleg@...hat.com>,
Elvira Khabirova <lineprinter@...linux.org>,
Eugene Syromyatnikov <evgsyr@...il.com>,
Dan Carpenter <dan.carpenter@...cle.com>,
Arnd Bergmann <arnd@...db.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-kernel-mentees@...ts.linuxfoundation.org,
linux-kernel@...r.kernel.org
Subject: Re: [Linux-kernel-mentees] [PATCH v2] ptrace: Prevent
kernel-infoleak in ptrace_get_syscall_info()
On Sat, Aug 01, 2020 at 02:06:46PM +0300, Dmitry V. Levin wrote:
> On Fri, Jul 31, 2020 at 10:08:41PM -0400, Peilin Ye wrote:
> > ptrace_get_syscall_info() is potentially copying uninitialized stack
> > memory to userspace, since the compiler may leave a 3-byte hole near the
> > beginning of `info`. Fix it by adding a padding field to `struct
> > ptrace_syscall_info`.
> >
> > Cc: stable@...r.kernel.org
> > Fixes: 201766a20e30 ("ptrace: add PTRACE_GET_SYSCALL_INFO request")
> > Suggested-by: Dan Carpenter <dan.carpenter@...cle.com>
> > Signed-off-by: Peilin Ye <yepeilin.cs@...il.com>
> > ---
> > Change in v2:
> > - Add a padding field to `struct ptrace_syscall_info`, instead of
> > doing memset() on `info`. (Suggested by Dmitry V. Levin
> > <ldv@...linux.org>)
> >
> > Reference: https://lwn.net/Articles/417989/
> >
> > $ # before:
> > $ pahole -C "ptrace_syscall_info" kernel/ptrace.o
> > struct ptrace_syscall_info {
> > __u8 op; /* 0 1 */
> >
> > /* XXX 3 bytes hole, try to pack */
> >
> > __u32 arch __attribute__((__aligned__(4))); /* 4 4 */
> > __u64 instruction_pointer; /* 8 8 */
> > __u64 stack_pointer; /* 16 8 */
> > union {
> > struct {
> > __u64 nr; /* 24 8 */
> > __u64 args[6]; /* 32 48 */
> > } entry; /* 24 56 */
> > struct {
> > __s64 rval; /* 24 8 */
> > __u8 is_error; /* 32 1 */
> > } exit; /* 24 16 */
> > struct {
> > __u64 nr; /* 24 8 */
> > __u64 args[6]; /* 32 48 */
> > /* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
> > __u32 ret_data; /* 80 4 */
> > } seccomp; /* 24 64 */
> > }; /* 24 64 */
> >
> > /* size: 88, cachelines: 2, members: 5 */
> > /* sum members: 85, holes: 1, sum holes: 3 */
> > /* forced alignments: 1, forced holes: 1, sum forced holes: 3 */
> > /* last cacheline: 24 bytes */
> > } __attribute__((__aligned__(8)));
> > $
> > $ # after:
> > $ pahole -C "ptrace_syscall_info" kernel/ptrace.o
> > struct ptrace_syscall_info {
> > __u8 op; /* 0 1 */
> > __u8 pad[3]; /* 1 3 */
> > __u32 arch __attribute__((__aligned__(4))); /* 4 4 */
> > __u64 instruction_pointer; /* 8 8 */
> > __u64 stack_pointer; /* 16 8 */
> > union {
> > struct {
> > __u64 nr; /* 24 8 */
> > __u64 args[6]; /* 32 48 */
> > } entry; /* 24 56 */
> > struct {
> > __s64 rval; /* 24 8 */
> > __u8 is_error; /* 32 1 */
> > } exit; /* 24 16 */
> > struct {
> > __u64 nr; /* 24 8 */
> > __u64 args[6]; /* 32 48 */
> > /* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
> > __u32 ret_data; /* 80 4 */
> > } seccomp; /* 24 64 */
> > }; /* 24 64 */
> >
> > /* size: 88, cachelines: 2, members: 6 */
> > /* forced alignments: 1 */
> > /* last cacheline: 24 bytes */
> > } __attribute__((__aligned__(8)));
> > $ _
> >
> > include/uapi/linux/ptrace.h | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
> > index a71b6e3b03eb..a518ba514bac 100644
> > --- a/include/uapi/linux/ptrace.h
> > +++ b/include/uapi/linux/ptrace.h
> > @@ -81,6 +81,7 @@ struct seccomp_metadata {
> >
> > struct ptrace_syscall_info {
> > __u8 op; /* PTRACE_SYSCALL_INFO_* */
> > + __u8 pad[3];
> > __u32 arch __attribute__((__aligned__(sizeof(__u32))));
> > __u64 instruction_pointer;
> > __u64 stack_pointer;
>
> Funnily enough, but in first editions of PTRACE_GET_SYSCALL_INFO
> patchset [1] this was looking very similar:
>
> +struct ptrace_syscall_info {
> + __u8 op; /* PTRACE_SYSCALL_INFO_* */
> + __u8 __pad0[3];
> + __u32 arch;
>
> But later we decided [2][3] to replace the pad with a hole.
>
> Note that the sole purpose of the __aligned__ attribute on the field that
> follows the hole is to guarantee that the hole has the same size across
> architectures. As this hole is being replaced back with a pad, that
> __aligned__ attribute is no longer needed and can be omitted along with
> adding the pad.
Ah, I see. I will remove that in v3.
Thank you,
Peilin Ye
> [1] https://lore.kernel.org/linux-api/20181125022150.46258a20@akathisia/
> [2] https://lore.kernel.org/linux-api/20181211162305.GA480@altlinux.org/
> [3] https://lore.kernel.org/linux-api/20181213171833.GA5240@altlinux.org/
>
>
> --
> ldv
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