[PATCH v4.9.z] pstore/ram: Write new dumps to start of recycled zones

[Kees Cook <keescook@chromium.org>]

From: Aleksandr Yashkin <a.yashkin@inango-systems.com>

[ Upstream commit 9e5f1c19800b808a37fb9815a26d382132c26c3d ]

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@inango-systems.com>
Signed-off-by: Nikolay Merinov <n.merinov@inango-systems.com>
Signed-off-by: Ariel Gilman <a.gilman@inango-systems.com>
Link: https://lore.kernel.org/r/20191223133816.28155-1-n.merinov@inango-systems.com
Fixes: 896fc1f0c4c6 ("pstore/ram: Switch to persistent_ram routines")
[kees: backport to v4.9]
Link: https://lore.kernel.org/stable/157831399811194@kroah.com
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 fs/pstore/ram.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
index 8b09271e5d66..a73959e6ae32 100644
--- a/fs/pstore/ram.c
+++ b/fs/pstore/ram.c
@@ -321,6 +321,17 @@ static int notrace ramoops_pstore_write_buf(enum pstore_type_id type,
 
 	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;
-- 
2.20.1


-- 
Kees Cook