[PATCH v2 1/3] accel/tcg: Make PageFlagsNodes' start and last immutable

[Ilya Leoshkevich <iii@linux.ibm.com>]

page_check_range() may race with pageflags_set_clear() as follows:

    T1                                     T2
    -------------------------------------  --------------------------------
                                           p = pageflags_find(start, last);
    interval_tree_remove(&p->itree, ...);
    p->itree.start = last + 1;
                                           if (start < p->itree.start) {
                                               ret = false;
    interval_tree_insert(&p->itree, ...);

leading to errors like

    fail indirect write 0x72f0a659aff0 (Bad address)

in vma-pthread test. I am able to reliably reproduce this on a machine
with 32 SMT threads as follows in about 25 seconds:

    jobs=32; \
    seq "$jobs" | \
        time -p parallel \
            --jobs="$jobs" \
            --halt=now,done=1 \
            --ungroup \
            '
                _={};
                while ./qemu-s390x tests/tcg/s390x-linux-user/vma-pthread; do
                    printf .;
                done
            '

Also wasmtime project reported a similar failure pattern in their CI [1]
with a similar reproducer [2].

There are other races like this. In general, region bounds mutating
underneath the reader are very hard to reason about. So fix this by
preventing mutations and creating copies instead. Use RCU guards in
readers to avoid uses-after-frees.

Now, when the reader finds a node, it may fearlessly access its fields
and be certain that at some point in time the respective region had the
respective bounds and permissions. The downside is slightly more
expensive mprotect(), but complexity reduction is worth it.

Lockless field accesses should probably be wrapped in qatomic_read(),
but this is a pre-existing issue, so do not change it here.

[1] https://github.com/bytecodealliance/wasmtime/issues/10000
[2] https://gist.github.com/alexcrichton/f14f23a892ffb9df2522754572d51b1c

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 accel/tcg/user-exec.c | 37 +++++++++++++++++++++----------------
 1 file changed, 21 insertions(+), 16 deletions(-)

diff --git a/accel/tcg/user-exec.c b/accel/tcg/user-exec.c
index 7704e4017dd..55761cacf65 100644
--- a/accel/tcg/user-exec.c
+++ b/accel/tcg/user-exec.c
@@ -239,13 +239,16 @@ void page_dump(FILE *f)
 
 int page_get_flags(vaddr address)
 {
-    PageFlagsNode *p = pageflags_find(address, address);
+    PageFlagsNode *p;
+
+    RCU_READ_LOCK_GUARD();
 
     /*
      * See util/interval-tree.c re lockless lookups: no false positives but
      * there are false negatives.  If we find nothing, retry with the mmap
      * lock acquired.
      */
+    p = pageflags_find(address, address);
     if (p) {
         return p->flags;
     }
@@ -301,15 +304,15 @@ static void pageflags_create_merge(vaddr start, vaddr last, int flags)
 
     if (prev) {
         if (next) {
-            prev->itree.last = next->itree.last;
+            pageflags_create(prev->itree.start, next->itree.last, flags);
             g_free_rcu(next, rcu);
         } else {
-            prev->itree.last = last;
+            pageflags_create(prev->itree.start, last, flags);
         }
-        interval_tree_insert(&prev->itree, &pageflags_root);
+        g_free_rcu(prev, rcu);
     } else if (next) {
-        next->itree.start = start;
-        interval_tree_insert(&next->itree, &pageflags_root);
+        pageflags_create(start, next->itree.last, flags);
+        g_free_rcu(next, rcu);
     } else {
         pageflags_create(start, last, flags);
     }
@@ -371,8 +374,8 @@ static bool pageflags_set_clear(vaddr start, vaddr last,
     if (set_flags != merge_flags) {
         if (p_start < start) {
             interval_tree_remove(&p->itree, &pageflags_root);
-            p->itree.last = start - 1;
-            interval_tree_insert(&p->itree, &pageflags_root);
+            pageflags_create(p_start, start - 1, p_flags);
+            g_free_rcu(p, rcu);
 
             if (last < p_last) {
                 if (merge_flags & PAGE_VALID) {
@@ -394,11 +397,11 @@ static bool pageflags_set_clear(vaddr start, vaddr last,
             }
             if (last < p_last) {
                 interval_tree_remove(&p->itree, &pageflags_root);
-                p->itree.start = last + 1;
-                interval_tree_insert(&p->itree, &pageflags_root);
+                pageflags_create(last + 1, p_last, p_flags);
                 if (merge_flags & PAGE_VALID) {
                     pageflags_create(start, last, merge_flags);
                 }
+                g_free_rcu(p, rcu);
             } else {
                 if (merge_flags & PAGE_VALID) {
                     p->flags = merge_flags;
@@ -419,8 +422,8 @@ static bool pageflags_set_clear(vaddr start, vaddr last,
     if (set_flags == p_flags) {
         if (start < p_start) {
             interval_tree_remove(&p->itree, &pageflags_root);
-            p->itree.start = start;
-            interval_tree_insert(&p->itree, &pageflags_root);
+            pageflags_create(start, p_last, p_flags);
+            g_free_rcu(p, rcu);
         }
         if (p_last < last) {
             start = p_last + 1;
@@ -432,8 +435,8 @@ static bool pageflags_set_clear(vaddr start, vaddr last,
     /* Maybe split out head and/or tail ranges with the original flags. */
     interval_tree_remove(&p->itree, &pageflags_root);
     if (p_start < start) {
-        p->itree.last = start - 1;
-        interval_tree_insert(&p->itree, &pageflags_root);
+        pageflags_create(p_start, start - 1, p_flags);
+        g_free_rcu(p, rcu);
 
         if (p_last < last) {
             goto restart;
@@ -442,8 +445,8 @@ static bool pageflags_set_clear(vaddr start, vaddr last,
             pageflags_create(last + 1, p_last, p_flags);
         }
     } else if (last < p_last) {
-        p->itree.start = last + 1;
-        interval_tree_insert(&p->itree, &pageflags_root);
+        pageflags_create(last + 1, p_last, p_flags);
+        g_free_rcu(p, rcu);
     } else {
         g_free_rcu(p, rcu);
         goto restart;
@@ -505,6 +508,8 @@ bool page_check_range(vaddr start, vaddr len, int flags)
         return false; /* wrap around */
     }
 
+    RCU_READ_LOCK_GUARD();
+
     locked = have_mmap_lock();
     while (true) {
         PageFlagsNode *p = pageflags_find(start, last);
-- 
2.54.0