linux-kernel - [PATCH] x86/speculation/l1tf: fix off-by-one error when warning that system has too much RAM

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Date:   Thu, 23 Aug 2018 15:44:18 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Cc:     "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, stable@...r.kernel.org,
        Christopher Snowhill <kode54@...il.com>,
        George Anchev <studio@...hev.net>
Subject: [PATCH] x86/speculation/l1tf: fix off-by-one error when warning that system has too much RAM

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 prefer 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), but it's more common and hopefully less error-prone to return the
first pfn that's over limit, so this patch changes that and updates the other
callers.

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

Reported-by: George Anchev <studio@...hev.net>
Reported-by: Christopher Snowhill <kode54@...il.com>
Fixes: 17dbca119312 ("x86/speculation/l1tf: Add sysfs reporting for l1tf")
Cc: stable@...r.kernel.org
Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
---
 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 a0a52274cb4a..c24297268ebc 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 02de3d6065c4..63a6f9fcaf20 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -923,7 +923,7 @@ 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 f40ab8185d94..1e95d57760cf 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -257,7 +257,7 @@ 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;
 }
-- 
2.18.0