On Thu 23-08-18 15:44:18, Vlastimil Babka wrote:
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@anchev.net Reported-by: Christopher Snowhill kode54@gmail.com Fixes: 17dbca119312 ("x86/speculation/l1tf: Add sysfs reporting for l1tf") Cc: stable@vger.kernel.org Signed-off-by: Vlastimil Babka vbabka@suse.cz
Yes this should be less error prone. Acked-by: Michal Hocko mhocko@suse.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 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