[PATCH v3 1/1] x86/boot/compressed: Fix avoiding memmap in physical KASLR

[Michal Clapinski <mclapinski@google.com>]

The intent of the code was to cancel KASLR if there are more than 4
memmap args. Unfortunately, it was only doing that if the memmap args
were comma delimited, not if they were entirely separate.

So it would disable physical KASLR for:
memmap=1G!4G,1G!5G,1G!6G,1G!7G,1G!8G
since the whole function is just called once and we hit the `if` at
the end of the function.

But it would not disable physical KASLR for:
memmap=1G!4G memmap=1G!5G memmap=1G!6G memmap=1G!7G memmap=1G!8G
The whole function would be called 5 times in total. On the 4th run the
last `if` would not trigger since `str` would be null. On the 5th run
we would exit early via the first `if`. That way the last `if` would
never trigger.

For the second input, the code would avoid the first 4 memmap regions
but not the last one (it could put the kernel there).

The new code disables physical KASLR for both of those inputs.

Signed-off-by: Michal Clapinski <mclapinski@google.com>
Suggested-by: Chris Li <chrisl@kernel.org>
Link: https://lore.kernel.org/all/CAF8kJuMvX31n8yNWn11bo1wCgXXOwOAp8HbYpSEBy94LR6phDA@mail.gmail.com/
Fixes: d52e7d5a952c ("x86/KASLR: Parse all 'memmap=' boot option entries")
---
v3: added a link in the description and a better explanation
v2: used Chris Li's snippet to change the flow of the function
---
 arch/x86/boot/compressed/kaslr.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index 3b0948ad449f..649264503ce6 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -162,14 +162,18 @@ static void mem_avoid_memmap(char *str)
 {
 	static int i;
 
-	if (i >= MAX_MEMMAP_REGIONS)
-		return;
-
-	while (str && (i < MAX_MEMMAP_REGIONS)) {
+	while (str) {
 		int rc;
 		u64 start, size;
-		char *k = strchr(str, ',');
+		char *k;
+
+		if (i >= MAX_MEMMAP_REGIONS) {
+			/* Too many memmap regions, disable physical KASLR. */
+			memmap_too_large = true;
+			return;
+		}
 
+		k = strchr(str, ',');
 		if (k)
 			*k++ = 0;
 
@@ -190,10 +194,6 @@ static void mem_avoid_memmap(char *str)
 		mem_avoid[MEM_AVOID_MEMMAP_BEGIN + i].size = size;
 		i++;
 	}
-
-	/* More than 4 memmaps, fail kaslr */
-	if ((i >= MAX_MEMMAP_REGIONS) && str)
-		memmap_too_large = true;
 }
 
 /* Store the number of 1GB huge pages which users specified: */
-- 
2.51.2.1026.g39e6a42477-goog