linux-hardening - [PATCH 2/2] xtensa: use XCHAL_NUM_AREGS as pt

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Message-Id: <20220306064435.256328-3-jcmvbkbc@gmail.com>
Date:   Sat,  5 Mar 2022 22:44:35 -0800
From:   Max Filippov <jcmvbkbc@...il.com>
To:     linux-xtensa@...ux-xtensa.org
Cc:     Chris Zankel <chris@...kel.net>, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org, Max Filippov <jcmvbkbc@...il.com>
Subject: [PATCH 2/2] xtensa: use XCHAL_NUM_AREGS as pt_regs::areg size

struct pt_regs is used to access both kernel and user exception frames.
User exception frames may contain up to XCHAL_NUM_AREG registers that
task creation and signal delivery code may access, but pt_regs::areg
array has only 16 entries that cover only the kernel exception frame.
This results in the following build error:

arch/xtensa/kernel/process.c: In function 'copy_thread':
arch/xtensa/kernel/process.c:262:52: error: array subscript 53 is above
           array bounds of 'long unsigned int[16]' [-Werror=array-bounds]
  262 |                                 put_user(regs->areg[caller_ars+1],

Change struct pt_regs::areg size to XCHAL_NUM_AREGS so that it covers
the whole user exception frame. Adjust task_pt_regs and drop additional
register copying code from copy_thread now that the whole user exception
stack frame is copied.

Reported-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Max Filippov <jcmvbkbc@...il.com>
---
 arch/xtensa/include/asm/ptrace.h |  7 +++----
 arch/xtensa/kernel/process.c     | 10 ----------
 2 files changed, 3 insertions(+), 14 deletions(-)

diff --git a/arch/xtensa/include/asm/ptrace.h b/arch/xtensa/include/asm/ptrace.h
index b109416dc07e..308f209a4740 100644
--- a/arch/xtensa/include/asm/ptrace.h
+++ b/arch/xtensa/include/asm/ptrace.h
@@ -44,6 +44,7 @@
 #ifndef __ASSEMBLY__
 
 #include <asm/coprocessor.h>
+#include <asm/core.h>
 
 /*
  * This struct defines the way the registers are stored on the
@@ -77,14 +78,12 @@ struct pt_regs {
 	/* current register frame.
 	 * Note: The ESF for kernel exceptions ends after 16 registers!
 	 */
-	unsigned long areg[16];
+	unsigned long areg[XCHAL_NUM_AREGS];
 };
 
-#include <asm/core.h>
-
 # define arch_has_single_step()	(1)
 # define task_pt_regs(tsk) ((struct pt_regs*) \
-	(task_stack_page(tsk) + KERNEL_STACK_SIZE - (XCHAL_NUM_AREGS-16)*4) - 1)
+	(task_stack_page(tsk) + KERNEL_STACK_SIZE) - 1)
 # define user_mode(regs) (((regs)->ps & 0x00000020)!=0)
 # define instruction_pointer(regs) ((regs)->pc)
 # define return_pointer(regs) (MAKE_PC_FROM_RA((regs)->areg[0], \
diff --git a/arch/xtensa/kernel/process.c b/arch/xtensa/kernel/process.c
index bd80df890b1e..e8bfbca5f001 100644
--- a/arch/xtensa/kernel/process.c
+++ b/arch/xtensa/kernel/process.c
@@ -232,10 +232,6 @@ int copy_thread(unsigned long clone_flags, unsigned long usp_thread_fn,
 		p->thread.ra = MAKE_RA_FOR_CALL(
 				(unsigned long)ret_from_fork, 0x1);
 
-		/* This does not copy all the regs.
-		 * In a bout of brilliance or madness,
-		 * ARs beyond a0-a15 exist past the end of the struct.
-		 */
 		*childregs = *regs;
 		childregs->areg[1] = usp;
 		childregs->areg[2] = 0;
@@ -265,14 +261,8 @@ int copy_thread(unsigned long clone_flags, unsigned long usp_thread_fn,
 			childregs->wmask = 1;
 			childregs->windowstart = 1;
 			childregs->windowbase = 0;
-		} else {
-			int len = childregs->wmask & ~0xf;
-			memcpy(&childregs->areg[XCHAL_NUM_AREGS - len/4],
-			       &regs->areg[XCHAL_NUM_AREGS - len/4], len);
 		}
 
-		childregs->syscall = regs->syscall;
-
 		if (clone_flags & CLONE_SETTLS)
 			childregs->threadptr = tls;
 	} else {
-- 
2.30.2