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

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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: 17dbca1193 ("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
This commit is contained in:
Vlastimil Babka 2018-08-23 15:44:18 +02:00 committed by Thomas Gleixner
parent 94d7a86c21
commit b0a182f875
3 changed files with 3 additions and 3 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

2
arch/x86/include/asm/processor.h
View file

@ -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);

2
arch/x86/mm/init.c
View file

@ -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.

2
arch/x86/mm/mmap.c
View file

@ -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;
}