Re: [Linux-kernel-mentees] [PATCH v2] ptrace: Prevent kernel-infoleak in ptrace_get_syscall_info()

[Peilin Ye]

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@xxxxxxxxxxxxxxx
> > Fixes: 201766a20e30 ("ptrace: add PTRACE_GET_SYSCALL_INFO request")
> > Suggested-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > Signed-off-by: Peilin Ye <yepeilin.cs@xxxxxxxxx>
> > ---
> > Change in v2:
> >     - Add a padding field to `struct ptrace_syscall_info`, instead of
> >       doing memset() on `info`. (Suggested by Dmitry V. Levin
> >       <ldv@xxxxxxxxxxxx>)
> > 
> > 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@xxxxxxxxxxxx/
> [3] https://lore.kernel.org/linux-api/20181213171833.GA5240@xxxxxxxxxxxx/
> 
> 
> -- 
> ldv