[tip:x86/fpu] x86/cpuid: Prevent out of bound access in do_clear_cpu_cap()

[tip-bot for Thomas Gleixner]

Commit-ID:  57b8b1a1856adaa849d02d547411a553a531022b
Gitweb:     https://git.kernel.org/tip/57b8b1a1856adaa849d02d547411a553a531022b
Author:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Wed, 18 Oct 2017 19:39:35 +0200
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Wed, 18 Oct 2017 20:03:34 +0200

x86/cpuid: Prevent out of bound access in do_clear_cpu_cap()

do_clear_cpu_cap() allocates a bitmap to keep track of disabled feature
dependencies. That bitmap is sized NCAPINTS * BITS_PER_INIT. The possible
'features' which can be handed in are larger than this, because after the
capabilities the bug 'feature' bits occupy another 32bit. Not really
obvious...

So clearing any of the misfeature bits, as 32bit does for the F00F bug,
accesses that bitmap out of bounds thereby corrupting the stack.

Size the bitmap proper and add a sanity check to catch accidental out of
bound access.

Fixes: 0b00de857a64 ("x86/cpuid: Add generic table for CPUID dependencies")
Reported-by: kernel test robot <xiaolong.ye@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Link: 20171018022023.GA12058@yexl-desktop">https://lkml.kernel.org/r/20171018022023.GA12058@yexl-desktop
---
 arch/x86/kernel/cpu/cpuid-deps.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index e48eb73..c1d4984 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -75,11 +75,17 @@ static inline void clear_feature(struct cpuinfo_x86 *c, unsigned int feature)
 		__clear_cpu_cap(c, feature);
 }
 
+/* Take the capabilities and the BUG bits into account */
+#define MAX_FEATURE_BITS ((NCAPINTS + NBUGINTS) * sizeof(u32) * 8)
+
 static void do_clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int feature)
 {
-	bool changed;
-	DECLARE_BITMAP(disable, NCAPINTS * sizeof(u32) * 8);
+	DECLARE_BITMAP(disable, MAX_FEATURE_BITS);
 	const struct cpuid_dep *d;
+	bool changed;
+
+	if (WARN_ON(feature >= MAX_FEATURE_BITS))
+		return;
 
 	clear_feature(c, feature);