x86/speculation/l1tf: Fix off-by-one error when warning that system has too much RAM

tehcaster · KAGA-KOKO · commit b0a182f87568 · 2018-08-24T09:51:14.000+02:00
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: 17dbca1 ("x86/speculation/l1tf: Add sysfs reporting for l1tf") Reported-by: George Anchev <studio@anchev.net> Reported-by: Christopher Snowhill <kode54@gmail.com> Signed-off-by: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: "H . Peter Anvin" <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andi Kleen <ak@linux.intel.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20180823134418.17008-1-vbabka@suse.cz
diff --git 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
@@ -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
@@ -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;
 }

Original file line number	Diff line number	Diff line change
`@@ -183,7 +183,7 @@ extern void cpu_detect(struct cpuinfo_x86 *c);`
`183`	`183`
`184`	`184`	`static inline unsigned long long l1tf_pfn_limit(void)`
`185`	`185`	`{`
`186`		`- return BIT_ULL(boot_cpu_data.x86_phys_bits - 1 - PAGE_SHIFT) - 1;`
	`186`	`+ return BIT_ULL(boot_cpu_data.x86_phys_bits - 1 - PAGE_SHIFT);`
`187`	`187`	`}`
`188`	`188`
`189`	`189`	`extern void early_cpu_init(void);`
Original file line number	Diff line number	Diff line change
`@@ -257,7 +257,7 @@ bool pfn_modify_allowed(unsigned long pfn, pgprot_t prot)`
`257`	`257`	`/* If it's real memory always allow */`
`258`	`258`	`if (pfn_valid(pfn))`
`259`	`259`	`return true;`
`260`		`- if (pfn > l1tf_pfn_limit() && !capable(CAP_SYS_ADMIN))`
	`260`	`+ if (pfn >= l1tf_pfn_limit() && !capable(CAP_SYS_ADMIN))`
`261`	`261`	`return false;`
`262`	`262`	`return true;`
`263`	`263`	`}`