linux-kernel - [PATCH 5/8] x86/kaslr: Simplify __process_mem

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Message-Id: <20200727215047.3341098-6-nivedita@alum.mit.edu>
Date:   Mon, 27 Jul 2020 17:50:44 -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 5/8] x86/kaslr: Simplify __process_mem_region

Get rid of unnecessary temporary variables and redundant tests in
__process_mem_region.

Fix one minor bug: in case of an overlap, the beginning of the region
should be used even if it is exactly image_size, not just strictly
larger.

Change type of minimum/image_size arguments in process_mem_region to
unsigned long. These actually can never be above 4G (even on x86_64),
and they're unsigned long in every other function except this one.

Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
---
 arch/x86/boot/compressed/kaslr.c | 50 ++++++++------------------------
 1 file changed, 12 insertions(+), 38 deletions(-)

diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index 758d78433f94..850e131121f7 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -622,42 +622,24 @@ static void __process_mem_region(struct mem_vector *entry,
 				 unsigned long image_size)
 {
 	struct mem_vector region, overlap;
-	unsigned long start_orig, end;
-	struct mem_vector cur_entry;
+	unsigned long region_end;
 
-	/* Ignore entries entirely below our minimum. */
-	if (entry->start + entry->size < minimum)
-		return;
-
-	/* Ignore entries above memory limit */
-	end = min(entry->size + entry->start, mem_limit);
-	if (entry->start >= end)
-		return;
-	cur_entry.start = entry->start;
-	cur_entry.size = end - entry->start;
-
-	region.start = cur_entry.start;
-	region.size = cur_entry.size;
+	/* Clamp region to minimum and memory limit. */
+	region.start = clamp_val(entry->start, minimum, mem_limit);
+	region_end = clamp_val(entry->start + entry->size, minimum, mem_limit);
 
 	/* Give up if slot area array is full. */
 	while (slot_area_index < MAX_SLOT_AREA) {
-		start_orig = region.start;
-
-		/* Potentially raise address to minimum location. */
-		if (region.start < minimum)
-			region.start = minimum;
-
 		/* Potentially raise address to meet alignment needs. */
 		region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);
 
 		/* Did we raise the address above the passed in memory entry? */
-		if (region.start > cur_entry.start + cur_entry.size)
+		if (region.start > region_end)
 			return;
 
-		/* Reduce size by any delta from the original address. */
-		region.size -= region.start - start_orig;
+		region.size = region_end - region.start;
 
-		/* Return if region can't contain decompressed kernel */
+		/* Return if region can't contain decompressed kernel. */
 		if (region.size < image_size)
 			return;
 
@@ -668,27 +650,19 @@ static void __process_mem_region(struct mem_vector *entry,
 		}
 
 		/* Store beginning of region if holds at least image_size. */
-		if (overlap.start > region.start + image_size) {
-			struct mem_vector beginning;
-
-			beginning.start = region.start;
-			beginning.size = overlap.start - region.start;
-			process_gb_huge_pages(&beginning, image_size);
+		if (overlap.start - region.start >= image_size) {
+			region.size = overlap.start - region.start;
+			process_gb_huge_pages(&region, image_size);
 		}
 
-		/* Return if overlap extends to or past end of region. */
-		if (overlap.start + overlap.size >= region.start + region.size)
-			return;
-
 		/* Clip off the overlapping region and start over. */
-		region.size -= overlap.start - region.start + overlap.size;
 		region.start = overlap.start + overlap.size;
 	}
 }
 
 static bool process_mem_region(struct mem_vector *region,
-			       unsigned long long minimum,
-			       unsigned long long image_size)
+			       unsigned long minimum,
+			       unsigned long image_size)
 {
 	int i;
 	/*
-- 
2.26.2