[PATCH] powerpc/signal32: Use struct_group() to zero spe regs

[Kees Cook]

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring fields.

Add a struct_group() for the spe registers so that memset() can correctly reason
about the size:

   In function 'fortify_memset_chk',
       inlined from 'restore_user_regs.part.0' at arch/powerpc/kernel/signal_32.c:539:3:
   >> include/linux/fortify-string.h:195:4: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
     195 |    __write_overflow_field();
         |    ^~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: kernel test robot <lkp at intel.com>
Signed-off-by: Kees Cook <keescook at chromium.org>
---
 arch/powerpc/include/asm/processor.h |  6 ++++--
 arch/powerpc/kernel/signal_32.c      | 14 +++++++++-----
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index e39bd0ff69f3..978a80308466 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -191,8 +191,10 @@ struct thread_struct {
 	int		used_vsr;	/* set if process has used VSX */
 #endif /* CONFIG_VSX */
 #ifdef CONFIG_SPE
-	unsigned long	evr[32];	/* upper 32-bits of SPE regs */
-	u64		acc;		/* Accumulator */
+	struct_group(spe,
+		unsigned long	evr[32];	/* upper 32-bits of SPE regs */
+		u64		acc;		/* Accumulator */
+	);
 	unsigned long	spefscr;	/* SPE & eFP status */
 	unsigned long	spefscr_last;	/* SPEFSCR value on last prctl
 					   call or trap return */
diff --git a/arch/powerpc/kernel/signal_32.c b/arch/powerpc/kernel/signal_32.c
index 00a9c9cd6d42..5e1664b501e4 100644
--- a/arch/powerpc/kernel/signal_32.c
+++ b/arch/powerpc/kernel/signal_32.c
@@ -527,16 +527,20 @@ static long restore_user_regs(struct pt_regs *regs,
 	regs_set_return_msr(regs, regs->msr & ~(MSR_FP | MSR_FE0 | MSR_FE1));
 
 #ifdef CONFIG_SPE
-	/* force the process to reload the spe registers from
-	   current->thread when it next does spe instructions */
+	/*
+	 * Force the process to reload the spe registers from
+	 * current->thread when it next does spe instructions.
+	 * Since this is user ABI, we must enforce the sizing.
+	 */
+	BUILD_BUG_ON(sizeof(current->thread.spe) != ELF_NEVRREG * sizeof(u32));
 	regs_set_return_msr(regs, regs->msr & ~MSR_SPE);
 	if (msr & MSR_SPE) {
 		/* restore spe registers from the stack */
-		unsafe_copy_from_user(current->thread.evr, &sr->mc_vregs,
-				      ELF_NEVRREG * sizeof(u32), failed);
+		unsafe_copy_from_user(&current->thread.spe, &sr->mc_vregs,
+				      sizeof(current->thread.spe), failed);
 		current->thread.used_spe = true;
 	} else if (current->thread.used_spe)
-		memset(current->thread.evr, 0, ELF_NEVRREG * sizeof(u32));
+		memset(&current->thread.spe, 0, sizeof(current->thread.spe));
 
 	/* Always get SPEFSCR back */
 	unsafe_get_user(current->thread.spefscr, (u32 __user *)&sr->mc_vregs + ELF_NEVRREG, failed);
-- 
2.30.2