[SRU][Bionic][PATCH 2/3] x86/speculation/l1tf: Fix off-by-one error when warning that system has too much RAM

[Joseph Salisbury]

From: Vlastimil Babka <vbabka at suse.cz>

BugLink: https://bugs.launchpad.net/bugs/1789834

Two users have reported [1] that they have an "extremely unlikely" system
with more than MAX_PA/2 memory and L1TF mitigation is not effective. In
fact it's a CPU with 36bits phys limit (64GB) and 32GB memory, but due to
holes in the e820 map, the main region is almost 500MB over the 32GB limit:

[    0.000000] BIOS-e820: [mem 0x0000000100000000-0x000000081effffff] usable

Suggestions to use 'mem=32G' to enable the L1TF mitigation while losing the
500MB revealed, that there's an off-by-one error in the check in
l1tf_select_mitigation().

l1tf_pfn_limit() returns the last usable pfn (inclusive) and the range
check in the mitigation path does not take this into account.

Instead of amending the range check, make l1tf_pfn_limit() return the first
PFN which is over the limit which is less error prone. Adjust the other
users accordingly.

[1] https://bugzilla.suse.com/show_bug.cgi?id=1105536

Fixes: 17dbca119312 ("x86/speculation/l1tf: Add sysfs reporting for l1tf")
Reported-by: George Anchev <studio at anchev.net>
Reported-by: Christopher Snowhill <kode54 at gmail.com>
Signed-off-by: Vlastimil Babka <vbabka at suse.cz>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Cc: "H . Peter Anvin" <hpa at zytor.com>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Andi Kleen <ak at linux.intel.com>
Cc: Dave Hansen <dave.hansen at intel.com>
Cc: Michal Hocko <mhocko at kernel.org>
Cc: stable at vger.kernel.org
Link: https://lkml.kernel.org/r/20180823134418.17008-1-vbabka@suse.cz

(cherry picked from commit b0a182f875689647b014bc01d36b340217792852)
Signed-off-by: Joseph Salisbury <joseph.salisbury at canonical.com>
---
 arch/x86/include/asm/processor.h | 2 +-
 arch/x86/mm/init.c               | 2 +-
 arch/x86/mm/mmap.c               | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 57c51ec..8c770dd 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -183,7 +183,7 @@ extern void cpu_detect(struct cpuinfo_x86 *c);
 
 static inline unsigned long long l1tf_pfn_limit(void)
 {
-	return BIT_ULL(boot_cpu_data.x86_phys_bits - 1 - PAGE_SHIFT) - 1;
+	return BIT_ULL(boot_cpu_data.x86_phys_bits - 1 - PAGE_SHIFT);
 }
 
 extern void early_cpu_init(void);
diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index 6b419ac..eef30c5 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -895,7 +895,7 @@ unsigned long max_swapfile_size(void)
 
 	if (boot_cpu_has_bug(X86_BUG_L1TF)) {
 		/* Limit the swap file size to MAX_PA/2 for L1TF workaround */
-		unsigned long long l1tf_limit = l1tf_pfn_limit() + 1;
+		unsigned long long l1tf_limit = l1tf_pfn_limit();
 		/*
 		 * We encode swap offsets also with 3 bits below those for pfn
 		 * which makes the usable limit higher.
diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index bfeabff..03f8ed2 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -253,7 +253,7 @@ bool pfn_modify_allowed(unsigned long pfn, pgprot_t prot)
 	/* If it's real memory always allow */
 	if (pfn_valid(pfn))
 		return true;
-	if (pfn > l1tf_pfn_limit() && !capable(CAP_SYS_ADMIN))
+	if (pfn >= l1tf_pfn_limit() && !capable(CAP_SYS_ADMIN))
 		return false;
 	return true;
 }
-- 
2.7.4