Commit 9d2e929c authored Feb 14, 2019 by Michael Ellerman Committed by Greg Kroah-Hartman Mar 23, 2019

powerpc/ptrace: Simplify vr_get/set() to avoid GCC warning



commit ca6d5149d2ad0a8d2f9c28cbe379802260a0a5e0 upstream.

GCC 8 warns about the logic in vr_get/set(), which with -Werror breaks
the build:

  In function ‘user_regset_copyin’,
      inlined from ‘vr_set’ at arch/powerpc/kernel/ptrace.c:628:9:
  include/linux/regset.h:295:4: error: ‘memcpy’ offset [-527, -529] is
  out of the bounds [0, 16] of object ‘vrsave’ with type ‘union
  <anonymous>’ [-Werror=array-bounds]
  arch/powerpc/kernel/ptrace.c: In function ‘vr_set’:
  arch/powerpc/kernel/ptrace.c:623:5: note: ‘vrsave’ declared here
     } vrsave;

This has been identified as a regression in GCC, see GCC bug 88273.

However we can avoid the warning and also simplify the logic and make
it more robust.

Currently we pass -1 as end_pos to user_regset_copyout(). This says
"copy up to the end of the regset".

The definition of the regset is:
	[REGSET_VMX] = {
		.core_note_type = NT_PPC_VMX, .n = 34,
		.size = sizeof(vector128), .align = sizeof(vector128),
		.active = vr_active, .get = vr_get, .set = vr_set
	},

The end is calculated as (n * size), ie. 34 * sizeof(vector128).

In vr_get/set() we pass start_pos as 33 * sizeof(vector128), meaning
we can copy up to sizeof(vector128) into/out-of vrsave.

The on-stack vrsave is defined as:
  union {
	  elf_vrreg_t reg;
	  u32 word;
  } vrsave;

And elf_vrreg_t is:
  typedef __vector128 elf_vrreg_t;

So there is no bug, but we rely on all those sizes lining up,
otherwise we would have a kernel stack exposure/overwrite on our
hands.

Rather than relying on that we can pass an explict end_pos based on
the sizeof(vrsave). The result should be exactly the same but it's
more obviously not over-reading/writing the stack and it avoids the
compiler warning.

Reported-by: Meelis Roos <mroos@linux.ee>
Reported-by: Mathieu Malaterre <malat@debian.org>
Cc: stable@vger.kernel.org
Tested-by: Mathieu Malaterre <malat@debian.org>
Tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 344996a8

arch/powerpc/kernel/ptrace.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -561,6 +561,7 @@ static int vr_get(struct task_struct target, const struct user_regset regset,
		/*
		* Copy out only the low-order word of vrsave.
		*/
		int start, end;
		union {
		elf_vrreg_t reg;
		u32 word;
		@@ -569,8 +570,10 @@ static int vr_get(struct task_struct target, const struct user_regset regset,

		vrsave.word = target->thread.vrsave;

		start = 33 * sizeof(vector128);
		end = start + sizeof(vrsave);
		ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, &vrsave,
		33 * sizeof(vector128), -1);
		start, end);
		}

		return ret;
		@@ -608,6 +611,7 @@ static int vr_set(struct task_struct target, const struct user_regset regset,
		/*
		* We use only the first word of vrsave.
		*/
		int start, end;
		union {
		elf_vrreg_t reg;
		u32 word;
		@@ -616,8 +620,10 @@ static int vr_set(struct task_struct target, const struct user_regset regset,

		vrsave.word = target->thread.vrsave;

		start = 33 * sizeof(vector128);
		end = start + sizeof(vrsave);
		ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &vrsave,
		33 * sizeof(vector128), -1);
		start, end);
		if (!ret)
		target->thread.vrsave = vrsave.word;
		}