linux-kernel - [PATCH v3 09/21] x86/kaslr: Drop some redundant checks from __process_mem

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Message-Id: <20200728225722.67457-10-nivedita@alum.mit.edu>
Date:   Tue, 28 Jul 2020 18:57:10 -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 09/21] x86/kaslr: Drop some redundant checks from __process_mem_region

Clip the start and end of the region to minimum and mem_limit prior to
the loop. region.start can only increase during the loop, so raising it
to minimum before the loop is enough.

A region that becomes empty due to this will get checked in
the first iteration of the loop.

Drop the check for overlap extending beyond the end of the region. This
will get checked in the next loop iteration anyway.

Rename end to region_end for symmetry with region.start.

Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
---
 arch/x86/boot/compressed/kaslr.c | 27 ++++++---------------------
 1 file changed, 6 insertions(+), 21 deletions(-)

diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index 8cc47faea56d..d074986e8061 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -623,34 +623,23 @@ static void __process_mem_region(struct mem_vector *entry,
 				 unsigned long image_size)
 {
 	struct mem_vector region, overlap;
-	unsigned long end;
+	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;
-
-	region.start = entry->start;
+	/* Enforce minimum and memory limit. */
+	region.start = max_t(unsigned long long, entry->start, minimum);
+	region_end = min(entry->start + entry->size, mem_limit);
 
 	/* Give up if slot area array is full. */
 	while (slot_area_index < MAX_SLOT_AREA) {
-		/* 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 > end)
+		if (region.start > region_end)
 			return;
 
 		/* Reduce size by any delta from the original address. */
-		region.size = end - region.start;
+		region.size = region_end - region.start;
 
 		/* Return if region can't contain decompressed kernel */
 		if (region.size < image_size)
@@ -668,10 +657,6 @@ static void __process_mem_region(struct mem_vector *entry,
 			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.start = overlap.start + overlap.size;
 	}
-- 
2.26.2