perf/core: Avoid put_page() when GUP fails

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.

Fixes: fc7ce9c74c3ad ("perf/core, x86: Add PERF_SAMPLE_PHYS_ADDR")
Signed-off-by: Greg Thelen <gthelen@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20211111021814.757086-1-gthelen@google.com

diff --git a/kernel/events/core.c b/kernel/events/core.c
index f2253ea729a2732b0261f32280f77f1d1a2ebefb..523106a506eed0a93afc92aea8ff8a9bc7ca4209 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7154,7 +7154,6 @@ 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;
@@ -7173,14 +7172,15 @@ 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_page_fast_only(virt, 0, &p))
+                       if (get_user_page_fast_only(virt, 0, &p)) {
                                phys_addr = page_to_phys(p) + virt % PAGE_SIZE;
+                               put_page(p);
+                       }
                        pagefault_enable();
                }
-
-               if (p)
-                       put_page(p);
        }
 
        return phys_addr;