netfs: Fix kernel BUG in netfs_limit_iter() for ITER_KVEC iterators

When a process crashes and the kernel writes a core dump to a 9P
filesystem, __kernel_write() creates an ITER_KVEC iterator. This
iterator reaches netfs_limit_iter() via netfs_unbuffered_write(), which
only handles ITER_FOLIOQ, ITER_BVEC and ITER_XARRAY iterator types,
hitting the BUG() for any other type.

Fix this by adding netfs_limit_kvec() following the same pattern as
netfs_limit_bvec(), since both kvec and bvec are simple segment arrays
with pointer and length fields. Dispatch it from netfs_limit_iter() when
the iterator type is ITER_KVEC.

Fixes: cae932d3aee5 ("netfs: Add func to calculate pagecount/size-limited span of an iterator")
Reported-by: syzbot+9c058f0d63475adc97fd@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9c058f0d63475adc97fd
Tested-by: syzbot+9c058f0d63475adc97fd@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
Link: https://patch.msgid.link/20260307090041.359870-1-kartikey406@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/netfs/iterator.c b/fs/netfs/iterator.c
index 72a435e5fc6da46f86fb032911bb8adb212e303f..154a14bb2d7f78c73c4c75ff5a4baa11e9f260b2 100644 (file)
--- a/fs/netfs/iterator.c
+++ b/fs/netfs/iterator.c
@@ -142,6 +142,47 @@ static size_t netfs_limit_bvec(const struct iov_iter *iter, size_t start_offset,
        return min(span, max_size);
 }
 
+/*
+ * Select the span of a kvec iterator we're going to use.  Limit it by both
+ * maximum size and maximum number of segments.  Returns the size of the span
+ * in bytes.
+ */
+static size_t netfs_limit_kvec(const struct iov_iter *iter, size_t start_offset,
+                              size_t max_size, size_t max_segs)
+{
+       const struct kvec *kvecs = iter->kvec;
+       unsigned int nkv = iter->nr_segs, ix = 0, nsegs = 0;
+       size_t len, span = 0, n = iter->count;
+       size_t skip = iter->iov_offset + start_offset;
+
+       if (WARN_ON(!iov_iter_is_kvec(iter)) ||
+           WARN_ON(start_offset > n) ||
+           n == 0)
+               return 0;
+
+       while (n && ix < nkv && skip) {
+               len = kvecs[ix].iov_len;
+               if (skip < len)
+                       break;
+               skip -= len;
+               n -= len;
+               ix++;
+       }
+
+       while (n && ix < nkv) {
+               len = min3(n, kvecs[ix].iov_len - skip, max_size);
+               span += len;
+               nsegs++;
+               ix++;
+               if (span >= max_size || nsegs >= max_segs)
+                       break;
+               skip = 0;
+               n -= len;
+       }
+
+       return min(span, max_size);
+}
+
 /*
  * Select the span of an xarray iterator we're going to use.  Limit it by both
  * maximum size and maximum number of segments.  It is assumed that segments
@@ -245,6 +286,8 @@ size_t netfs_limit_iter(const struct iov_iter *iter, size_t start_offset,
                return netfs_limit_bvec(iter, start_offset, max_size, max_segs);
        if (iov_iter_is_xarray(iter))
                return netfs_limit_xarray(iter, start_offset, max_size, max_segs);
+       if (iov_iter_is_kvec(iter))
+               return netfs_limit_kvec(iter, start_offset, max_size, max_segs);
        BUG();
 }
 EXPORT_SYMBOL(netfs_limit_iter);