[PATCH] arm64: kaslr: Add 2MB correction for aligning kernel image

[Srinivas Ramana]

From: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>

If kernel image extends across alignment boundary, existing
code increases the KASLR offset by size of kernel image. The
offset is masked after resizing. There are cases, where after
masking, we may still have kernel image extending across
boundary. This eventually results in only 2MB block getting
mapped while creating the page tables. This results in data aborts
while accessing unmapped regions during second relocation (with
kaslr offset) in __primary_switch. To fix this problem, add a
2MB correction to offset along with the correction of kernel
image size, before applying mask.

For example consider below case, where kernel image still crosses
1GB alignment boundary, after masking the offset, which is fixed
by adding 2MB correction.

SWAPPER_TABLE_SHIFT = 30
Swapper using section maps with section size 2MB.
CONFIG_PGTABLE_LEVELS = 3
VA_BITS = 39

_text  : 0xffffff8008080000
_end   : 0xffffff800aa1b000
offset : 0x1f35600000
mask = ((1UL << (VA_BITS - 2)) - 1) & ~(SZ_2M - 1)

(_text + offset) >> SWAPPER_TABLE_SHIFT = 0x3fffffe7c
(_end + offset) >> SWAPPER_TABLE_SHIFT  = 0x3fffffe7d

offset after existing correction (before mask) = 0x1f37f9b000
(_text + offset) >> SWAPPER_TABLE_SHIFT = 0x3fffffe7d
(_end + offset) >> SWAPPER_TABLE_SHIFT  = 0x3fffffe7d

offset (after mask) = 0x1f37e00000
(_text + offset) >> SWAPPER_TABLE_SHIFT = 0x3fffffe7c
(_end + offset) >> SWAPPER_TABLE_SHIFT  = 0x3fffffe7d

new offset w/ 2MB correction (before mask) = 0x1f37819b00
new offset w/ 2MB correction (after mask) = 0x1f38000000
(_text + offset) >> SWAPPER_TABLE_SHIFT = 0x3fffffe7d
(_end + offset) >> SWAPPER_TABLE_SHIFT  = 0x3fffffe7d

Fixes: f80fb3a3d508 ("arm64: add support for kernel ASLR")
Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
Signed-off-by: Srinivas Ramana <sramana@xxxxxxxxxxxxxx>
---
 arch/arm64/kernel/kaslr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
index 769f24ef628c..7b8af985e497 100644
--- a/arch/arm64/kernel/kaslr.c
+++ b/arch/arm64/kernel/kaslr.c
@@ -135,7 +135,7 @@ u64 __init kaslr_early_init(u64 dt_phys, u64 modulo_offset)
 	 */
 	if ((((u64)_text + offset + modulo_offset) >> SWAPPER_TABLE_SHIFT) !=
 	    (((u64)_end + offset + modulo_offset) >> SWAPPER_TABLE_SHIFT))
-		offset = (offset + (u64)(_end - _text)) & mask;
+		offset = (offset + (u64)(_end - _text) + SZ_2M) & mask;
 
 	if (IS_ENABLED(CONFIG_KASAN))
 		/*
-- 
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