[PATCH 3.16 004/148] pstore/ram: Write new dumps to start of recycled zones

[Ben Hutchings]

3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Aleksandr Yashkin <a.yashkin@xxxxxxxxxxxxxxxxxx>

commit 9e5f1c19800b808a37fb9815a26d382132c26c3d upstream.

The ram_core.c routines treat przs as circular buffers. When writing a
new crash dump, the old buffer needs to be cleared so that the new dump
doesn't end up in the wrong place (i.e. at the end).

The solution to this problem is to reset the circular buffer state before
writing a new Oops dump.

Signed-off-by: Aleksandr Yashkin <a.yashkin@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Nikolay Merinov <n.merinov@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Ariel Gilman <a.gilman@xxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20191223133816.28155-1-n.merinov@xxxxxxxxxxxxxxxxxx
Fixes: 896fc1f0c4c6 ("pstore/ram: Switch to persistent_ram routines")
[kees: backport to v4.9]
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/pstore/ram.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

--- a/fs/pstore/ram.c
+++ b/fs/pstore/ram.c
@@ -273,6 +273,17 @@ static int notrace ramoops_pstore_write_
 
 	prz = cxt->przs[cxt->dump_write_cnt];
 
+	/*
+	 * Since this is a new crash dump, we need to reset the buffer in
+	 * case it still has an old dump present. Without this, the new dump
+	 * will get appended, which would seriously confuse anything trying
+	 * to check dump file contents. Specifically, ramoops_read_kmsg_hdr()
+	 * expects to find a dump header in the beginning of buffer data, so
+	 * we must to reset the buffer values, in order to ensure that the
+	 * header will be written to the beginning of the buffer.
+	 */
+	persistent_ram_zap(prz);
+
 	hlen = ramoops_write_kmsg_hdr(prz, compressed);
 	if (size + hlen > prz->buffer_size)
 		size = prz->buffer_size - hlen;