[tip: x86/urgent] x86/mtrr: Correct the range check before performing MTRR type lookups

[tip-bot2 for Ying-Tsun Huang]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     cb7f4a8b1fb426a175d1708f05581939c61329d4
Gitweb:        https://git.kernel.org/tip/cb7f4a8b1fb426a175d1708f05581939c61329d4
Author:        Ying-Tsun Huang <ying-tsun.huang@xxxxxxx>
AuthorDate:    Tue, 15 Dec 2020 15:07:20 +08:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Wed, 06 Jan 2021 13:01:13 +01:00

x86/mtrr: Correct the range check before performing MTRR type lookups

In mtrr_type_lookup(), if the input memory address region is not in the
MTRR, over 4GB, and not over the top of memory, a write-back attribute
is returned. These condition checks are for ensuring the input memory
address region is actually mapped to the physical memory.

However, if the end address is just aligned with the top of memory,
the condition check treats the address is over the top of memory, and
write-back attribute is not returned.

And this hits in a real use case with NVDIMM: the nd_pmem module tries
to map NVDIMMs as cacheable memories when NVDIMMs are connected. If a
NVDIMM is the last of the DIMMs, the performance of this NVDIMM becomes
very low since it is aligned with the top of memory and its memory type
is uncached-minus.

Move the input end address change to inclusive up into
mtrr_type_lookup(), before checking for the top of memory in either
mtrr_type_lookup_{variable,fixed}() helpers.

 [ bp: Massage commit message. ]

Fixes: 0cc705f56e40 ("x86/mm/mtrr: Clean up mtrr_type_lookup()")
Signed-off-by: Ying-Tsun Huang <ying-tsun.huang@xxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lkml.kernel.org/r/20201215070721.4349-1-ying-tsun.huang@xxxxxxx
---
 arch/x86/kernel/cpu/mtrr/generic.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
index 23ad8e9..a29997e 100644
--- a/arch/x86/kernel/cpu/mtrr/generic.c
+++ b/arch/x86/kernel/cpu/mtrr/generic.c
@@ -167,9 +167,6 @@ static u8 mtrr_type_lookup_variable(u64 start, u64 end, u64 *partial_end,
 	*repeat = 0;
 	*uniform = 1;
 
-	/* Make end inclusive instead of exclusive */
-	end--;
-
 	prev_match = MTRR_TYPE_INVALID;
 	for (i = 0; i < num_var_ranges; ++i) {
 		unsigned short start_state, end_state, inclusive;
@@ -261,6 +258,9 @@ u8 mtrr_type_lookup(u64 start, u64 end, u8 *uniform)
 	int repeat;
 	u64 partial_end;
 
+	/* Make end inclusive instead of exclusive */
+	end--;
+
 	if (!mtrr_state_set)
 		return MTRR_TYPE_INVALID;