Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 23 Feb 2022 01:30:23 +0000
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>
From:   Alexander Lobakin <alobakin@pm.me>
Cc:     Alexander Lobakin <alobakin@pm.me>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Mike Rapoport <rppt@kernel.org>,
        Davidlohr Bueso <dbueso@suse.de>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Liam Howlett <liam.howlett@oracle.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        Atsushi Nemoto <anemo@mba.ocn.ne.jp>,
        linux-mips@vger.kernel.org, stable@vger.kernel.org,
        linux-kernel@vger.kernel.org
Reply-To: Alexander Lobakin <alobakin@pm.me>
Subject: [PATCH mips-fixes] MIPS: fix fortify panic when copying asm exception handlers
Message-ID: <20220223012338.262041-1-alobakin@pm.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

With KCFLAGS=3D"-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.

Fixes: e01402b115cc ("More AP / SP bits for the 34K, the Malta bits and thi=
ngs. Still wants")
Fixes: c65a5480ff29 ("[MIPS] Fix potential latency problem due to non-atomi=
c cpu_wait.")
Cc: stable@vger.kernel.org # 3.10+
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
---
 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 bb36a400203d..8c56b862fd9c 100644
--- a/arch/mips/include/asm/setup.h
+++ b/arch/mips/include/asm/setup.h
@@ -16,7 +16,7 @@ static inline void setup_8250_early_printk_port(unsigned =
long base,
 =09unsigned int reg_shift, unsigned int timeout) {}
 #endif

-extern void set_handler(unsigned long offset, void *addr, unsigned long le=
n);
+void set_handler(unsigned long offset, const void *addr, unsigned long len=
);
 extern void set_uncached_handler(unsigned long offset, void *addr, unsigne=
d long len);

 typedef void (*vi_handler_t)(void);
diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
index a486486b2355..246c6a6b0261 100644
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -2091,19 +2091,19 @@ static void *set_vi_srs_handler(int n, vi_handler_t=
 addr, int srs)
 =09=09 * If no shadow set is selected then use the default handler
 =09=09 * that does normal register saving and standard interrupt exit
 =09=09 */
-=09=09extern char except_vec_vi, except_vec_vi_lui;
-=09=09extern char except_vec_vi_ori, except_vec_vi_end;
-=09=09extern char rollback_except_vec_vi;
-=09=09char *vec_start =3D using_rollback_handler() ?
-=09=09=09&rollback_except_vec_vi : &except_vec_vi;
+=09=09extern const u8 except_vec_vi[], except_vec_vi_lui[];
+=09=09extern const u8 except_vec_vi_ori[], except_vec_vi_end[];
+=09=09extern const u8 rollback_except_vec_vi[];
+=09=09const u8 *vec_start =3D using_rollback_handler() ?
+=09=09=09=09      rollback_except_vec_vi : except_vec_vi;
 #if defined(CONFIG_CPU_MICROMIPS) || defined(CONFIG_CPU_BIG_ENDIAN)
-=09=09const int lui_offset =3D &except_vec_vi_lui - vec_start + 2;
-=09=09const int ori_offset =3D &except_vec_vi_ori - vec_start + 2;
+=09=09const int lui_offset =3D except_vec_vi_lui - vec_start + 2;
+=09=09const int ori_offset =3D except_vec_vi_ori - vec_start + 2;
 #else
-=09=09const int lui_offset =3D &except_vec_vi_lui - vec_start;
-=09=09const int ori_offset =3D &except_vec_vi_ori - vec_start;
+=09=09const int lui_offset =3D except_vec_vi_lui - vec_start;
+=09=09const int ori_offset =3D except_vec_vi_ori - vec_start;
 #endif
-=09=09const int handler_len =3D &except_vec_vi_end - vec_start;
+=09=09const int handler_len =3D except_vec_vi_end - vec_start;

 =09=09if (handler_len > VECTORSPACING) {
 =09=09=09/*
@@ -2311,7 +2311,7 @@ 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 siz=
e)
 {
 #ifdef CONFIG_CPU_MICROMIPS
 =09memcpy((void *)(ebase + offset), ((unsigned char *)addr - 1), size);
--
2.35.1