selftest/vm: verify remap destination address in mremap_test

Because mremap does not have a MAP_FIXED_NOREPLACE flag, it can destroy
existing mappings.  This causes a segfault when regions such as text are
remapped and the permissions are changed.

Verify the requested mremap destination address does not overlap any
existing mappings by using mmap's MAP_FIXED_NOREPLACE flag.  Keep
incrementing the destination address until a valid mapping is found or
fail the current test once the max address is reached.

Link: https://lkml.kernel.org/r/20220420215721.4868-2-sidhartha.kumar@oracle.com
Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/vm/mremap_test.c

@@ -10,6 +10,7 @@
 #include <string.h>
 #include <sys/mman.h>
 #include <time.h>
+#include <stdbool.h>
 
 #include "../kselftest.h"
 
@@ -64,6 +65,30 @@ enum {
 	.expect_failure = should_fail				\
 }
 
+/*
+ * Returns false if the requested remap region overlaps with an
+ * existing mapping (e.g text, stack) else returns true.
+ */
+static bool is_remap_region_valid(void *addr, unsigned long long size)
+{
+	void *remap_addr = NULL;
+	bool ret = true;
+
+	/* Use MAP_FIXED_NOREPLACE flag to ensure region is not mapped */
+	remap_addr = mmap(addr, size, PROT_READ | PROT_WRITE,
+					 MAP_FIXED_NOREPLACE | MAP_ANONYMOUS | MAP_SHARED,
+					 -1, 0);
+
+	if (remap_addr == MAP_FAILED) {
+		if (errno == EEXIST)
+			ret = false;
+	} else {
+		munmap(remap_addr, size);
+	}
+
+	return ret;
+}
+
 /* Returns mmap_min_addr sysctl tunable from procfs */
 static unsigned long long get_mmap_min_addr(void)
 {
@@ -179,6 +204,17 @@ static long long remap_region(struct config c, unsigned int threshold_mb,
 	if (!((unsigned long long) addr & c.dest_alignment))
 		addr = (void *) ((unsigned long long) addr | c.dest_alignment);
 
+	/* Don't destroy existing mappings unless expected to overlap */
+	while (!is_remap_region_valid(addr, c.region_size) && !c.overlapping) {
+		/* Check for unsigned overflow */
+		if (addr + c.dest_alignment < addr) {
+			ksft_print_msg("Couldn't find a valid region to remap to\n");
+			ret = -1;
+			goto out;
+		}
+		addr += c.dest_alignment;
+	}
+
 	clock_gettime(CLOCK_MONOTONIC, &t_start);
 	dest_addr = mremap(src_addr, c.region_size, c.region_size,
 					  MREMAP_MAYMOVE|MREMAP_FIXED, (char *) addr);