linux-kernel - [PATCH v3 16/21] x86/kaslr: Drop unnecessary alignment in find_random_virt

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Date:   Tue, 28 Jul 2020 18:57:17 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Kees Cook <keescook@...omium.org>, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH v3 16/21] x86/kaslr: Drop unnecessary alignment in find_random_virt_addr

Drop unnecessary alignment of image_size to CONFIG_PHYSICAL_ALIGN in
find_random_virt_addr, it cannot change the result: the largest valid
slot is the largest n that satisfies

  minimum + n * CONFIG_PHYSICAL_ALIGN + image_size <= KERNEL_IMAGE_SIZE

(since minimum is already aligned) and so n is equal to

  (KERNEL_IMAGE_SIZE - minimum - image_size) / CONFIG_PHYSICAL_ALIGN

even if image_size is not aligned to CONFIG_PHYSICAL_ALIGN.

Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
---
 arch/x86/boot/compressed/kaslr.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index 2f43b0d7051e..4fd60eff048f 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -824,16 +824,12 @@ static unsigned long find_random_virt_addr(unsigned long minimum,
 {
 	unsigned long slots, random_addr;
 
-	/* Align image_size for easy slot calculations. */
-	image_size = ALIGN(image_size, CONFIG_PHYSICAL_ALIGN);
-
 	/*
 	 * There are how many CONFIG_PHYSICAL_ALIGN-sized slots
 	 * that can hold image_size within the range of minimum to
 	 * KERNEL_IMAGE_SIZE?
 	 */
-	slots = (KERNEL_IMAGE_SIZE - minimum - image_size) /
-		 CONFIG_PHYSICAL_ALIGN + 1;
+	slots = 1 + (KERNEL_IMAGE_SIZE - minimum - image_size) / CONFIG_PHYSICAL_ALIGN;
 
 	random_addr = kaslr_get_random_long("Virtual") % slots;
 
-- 
2.26.2