[PATCH AUTOSEL 5.4 138/330] x86/pkeys: Add check for pkey "overflow"

Sasha Levin
2020-09-18T03:06:56Z

From: Dave Hansen <dave.hansen@linux.intel.com>

[ Upstream commit 16171bffc829272d5e6014bad48f680cb50943d9 ]

Alex Shi reported the pkey macros above arch_set_user_pkey_access()
to be unused. They are unused, and even refer to a nonexistent
CONFIG option.

But, they might have served a good use, which was to ensure that
the code does not try to set values that would not fit in the
PKRU register. As it stands, a too-large 'pkey' value would
be likely to silently overflow the u32 new_pkru_bits.

Add a check to look for overflows. Also add a comment to remind
any future developer to closely examine the types used to store
pkey values if arch_max_pkey() ever changes.

This boots and passes the x86 pkey selftests.

Reported-by: Alex Shi <alex.shi@linux.alibaba.com>
Signed-off-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200122165346.AD4DA150@viggo.jf.intel.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/include/asm/pkeys.h | 5 +++++
arch/x86/kernel/fpu/xstate.c | 9 +++++++--
2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
index 19b137f1b3beb..2ff9b98812b76 100644
--- a/arch/x86/include/asm/pkeys.h
+++ b/arch/x86/include/asm/pkeys.h
@@ -4,6 +4,11 @@

#define ARCH_DEFAULT_PKEY 0

+/*
+ * If more than 16 keys are ever supported, a thorough audit
+ * will be necessary to ensure that the types that store key
+ * numbers and masks have sufficient capacity.
+ */
#define arch_max_pkey() (boot_cpu_has(X86_FEATURE_OSPKE) ? 16 : 1)

extern int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 755eb26cbec04..735d1f1bbabc7 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -895,8 +895,6 @@ const void *get_xsave_field_ptr(int xfeature_nr)

#ifdef CONFIG_ARCH_HAS_PKEYS

-#define NR_VALID_PKRU_BITS (CONFIG_NR_PROTECTION_KEYS * 2)
-#define PKRU_VALID_MASK (NR_VALID_PKRU_BITS - 1)
/*
* This will go out and modify PKRU register to set the access
* rights for @pkey to @init_val.
@@ -915,6 +913,13 @@ int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
if (!boot_cpu_has(X86_FEATURE_OSPKE))
return -EINVAL;

+ /*
+ * This code should only be called with valid 'pkey'
+ * values originating from in-kernel users. Complain
+ * if a bad value is observed.
+ */
+ WARN_ON_ONCE(pkey >= arch_max_pkey());
+
/* Set the bits we need in PKRU: */
if (init_val & PKEY_DISABLE_ACCESS)
new_pkru_bits |= PKRU_AD_BIT;
--
2.25.1