perf/core: Avoid put_page() when GUP fails

commit 4716023a8f6a0f4a28047f14dd7ebdc319606b84 upstream.

PEBS PERF_SAMPLE_PHYS_ADDR events use perf_virt_to_phys() to convert PMU sampled virtual addresses to physical using get_user_page_fast_only() and page_to_phys().

Some get_user_page_fast_only() error cases return false, indicating no page reference, but still initialize the output page pointer with an unreferenced page. In these error cases perf_virt_to_phys() calls put_page(). This causes page reference count underflow, which can lead to unintentional page sharing.

Fix perf_virt_to_phys() to only put_page() if get_user_page_fast_only() returns a referenced page.

---
 kernel/events/core.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

index 4a8c3f5313f9..e6e11b598438 100644
@@ -6424,7 +6424,6 @@ void perf_output_sample(struct perf_output_handle *handle,
 static u64 perf_virt_to_phys(u64 virt)
 {
 	u64 phys_addr = 0;
-	struct page *p = NULL;
 
 	if (!virt)
 		return 0;

@@ -6443,14 +6442,15 @@ static u64 perf_virt_to_phys(u64 virt)
 		 * If failed, leave phys_addr as 0.
 		 */
 		if (current->mm != NULL) {
+			struct page *p;
+
 			pagefault_disable();
-			if (__get_user_pages_fast(virt, 1, 0, &p) == 1)
+			if (__get_user_pages_fast(virt, 1, 0, &p) == 1) {
 				phys_addr = page_to_phys(p) + virt % PAGE_SIZE;
+				put_page(p);
+			}
 			pagefault_enable();
 		}
-
-		if (p)
-			put_page(p);
 	}
 
 	return phys_addr;

2.34.0.rc2.393.gf8c9666880-goog