linux-kernel - [PATCH AUTOSEL 4.14 16/22] MIPS: fix fortify panic when copying asm exception handlers

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Message-Id: <20220401144729.1955554-16-sashal@kernel.org>
Date:   Fri,  1 Apr 2022 10:47:23 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Alexander Lobakin <alobakin@...me>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Sasha Levin <sashal@...nel.org>, ebiederm@...ssion.com,
        akpm@...ux-foundation.org, dbueso@...e.de, liam.howlett@...cle.com,
        f.fainelli@...il.com, linux-mips@...r.kernel.org
Subject: [PATCH AUTOSEL 4.14 16/22] MIPS: fix fortify panic when copying asm exception handlers

From: Alexander Lobakin <alobakin@...me>

[ Upstream commit d17b66417308996e7e64b270a3c7f3c1fbd4cfc8 ]

With KCFLAGS="-O3", I was able to trigger a fortify-source
memcpy() overflow panic on set_vi_srs_handler().
Although O3 level is not supported in the mainline, under some
conditions that may've happened with any optimization settings,
it's just a matter of inlining luck. The panic itself is correct,
more precisely, 50/50 false-positive and not at the same time.
>From the one side, no real overflow happens. Exception handler
defined in asm just gets copied to some reserved places in the
memory.
But the reason behind is that C code refers to that exception
handler declares it as `char`, i.e. something of 1 byte length.
It's obvious that the asm function itself is way more than 1 byte,
so fortify logics thought we are going to past the symbol declared.
The standard way to refer to asm symbols from C code which is not
supposed to be called from C is to declare them as
`extern const u8[]`. This is fully correct from any point of view,
as any code itself is just a bunch of bytes (including 0 as it is
for syms like _stext/_etext/etc.), and the exact size is not known
at the moment of compilation.
Adjust the type of the except_vec_vi_*() and related variables.
Make set_handler() take `const` as a second argument to avoid
cast-away warnings and give a little more room for optimization.

Signed-off-by: Alexander Lobakin <alobakin@...me>
Signed-off-by: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/mips/include/asm/setup.h |  2 +-
 arch/mips/kernel/traps.c      | 22 +++++++++++-----------
 2 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/arch/mips/include/asm/setup.h b/arch/mips/include/asm/setup.h
index d49d247d48a1..d48a5f18a267 100644
--- a/arch/mips/include/asm/setup.h
+++ b/arch/mips/include/asm/setup.h
@@ -14,7 +14,7 @@ static inline void setup_8250_early_printk_port(unsigned long base,
 	unsigned int reg_shift, unsigned int timeout) {}
 #endif
 
-extern void set_handler(unsigned long offset, void *addr, unsigned long len);
+void set_handler(unsigned long offset, const void *addr, unsigned long len);
 extern void set_uncached_handler(unsigned long offset, void *addr, unsigned long len);
 
 typedef void (*vi_handler_t)(void);
diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
index 4a23d89e251c..abbc64788008 100644
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -2017,19 +2017,19 @@ static void *set_vi_srs_handler(int n, vi_handler_t addr, int srs)
 		 * If no shadow set is selected then use the default handler
 		 * that does normal register saving and standard interrupt exit
 		 */
-		extern char except_vec_vi, except_vec_vi_lui;
-		extern char except_vec_vi_ori, except_vec_vi_end;
-		extern char rollback_except_vec_vi;
-		char *vec_start = using_rollback_handler() ?
-			&rollback_except_vec_vi : &except_vec_vi;
+		extern const u8 except_vec_vi[], except_vec_vi_lui[];
+		extern const u8 except_vec_vi_ori[], except_vec_vi_end[];
+		extern const u8 rollback_except_vec_vi[];
+		const u8 *vec_start = using_rollback_handler() ?
+				      rollback_except_vec_vi : except_vec_vi;
 #if defined(CONFIG_CPU_MICROMIPS) || defined(CONFIG_CPU_BIG_ENDIAN)
-		const int lui_offset = &except_vec_vi_lui - vec_start + 2;
-		const int ori_offset = &except_vec_vi_ori - vec_start + 2;
+		const int lui_offset = except_vec_vi_lui - vec_start + 2;
+		const int ori_offset = except_vec_vi_ori - vec_start + 2;
 #else
-		const int lui_offset = &except_vec_vi_lui - vec_start;
-		const int ori_offset = &except_vec_vi_ori - vec_start;
+		const int lui_offset = except_vec_vi_lui - vec_start;
+		const int ori_offset = except_vec_vi_ori - vec_start;
 #endif
-		const int handler_len = &except_vec_vi_end - vec_start;
+		const int handler_len = except_vec_vi_end - vec_start;
 
 		if (handler_len > VECTORSPACING) {
 			/*
@@ -2249,7 +2249,7 @@ void per_cpu_trap_init(bool is_boot_cpu)
 }
 
 /* Install CPU exception handler */
-void set_handler(unsigned long offset, void *addr, unsigned long size)
+void set_handler(unsigned long offset, const void *addr, unsigned long size)
 {
 #ifdef CONFIG_CPU_MICROMIPS
 	memcpy((void *)(ebase + offset), ((unsigned char *)addr - 1), size);
-- 
2.34.1