[PATCH 4.17 124/324] netfilter: nf_log: fix uninit read in nf_log_proc_dostring

[Greg Kroah-Hartman]

4.17-stable review patch.  If anyone has any objections, please let me know.

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

From: Jann Horn <jannh@xxxxxxxxxx>

[ Upstream commit dffd22aed2aa1e804bccf19b30a421e89ee2ae61 ]

When proc_dostring() is called with a non-zero offset in strict mode, it
doesn't just write to the ->data buffer, it also reads. Make sure it
doesn't read uninitialized data.

Fixes: c6ac37d8d884 ("netfilter: nf_log: fix error on write NONE to [...]")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/netfilter/nf_log.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/net/netfilter/nf_log.c
+++ b/net/netfilter/nf_log.c
@@ -439,6 +439,10 @@ static int nf_log_proc_dostring(struct c
 	if (write) {
 		struct ctl_table tmp = *table;
 
+		/* proc_dostring() can append to existing strings, so we need to
+		 * initialize it as an empty string.
+		 */
+		buf[0] = '\0';
 		tmp.data = buf;
 		r = proc_dostring(&tmp, write, buffer, lenp, ppos);
 		if (r)