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Subject[PATCH v3] printk: Fix discarding of records
Date
Found another buffer overflow in this code that was introduced by
e3756477aec028427fec767957c0d4b6cfb87208 trying to solve a related overflow.

strace still shows a problem:

syslog(0x3, 0x7fffd65375d0, 0x1000) = 4107

The first record output was in the middle of a LOG_CONT line:

<4>[ 2.974999] 0x0000000000000500-0x000000000000052f SystemIO conflicts with Region \GPIO 1 (20130328/utaddress-251)

This happens because when discarding records to be less than size, the first
line may expand due to there being no previous record. So we must use prev = 0
to calculate if it fits or we should continue discarding.

v3: fix whitespace
v2: fix loop properly

CC: stable@vger.kernel.org
Signed-off-by: Debabrata Banerjee <dbanerje@akamai.com>
---
kernel/printk/printk.c | 18 +++++++++++++-----
1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b1d255f..e95c7b4 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1063,13 +1063,17 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
seq = clear_seq;
idx = clear_idx;
prev = 0;
- while (len > size && seq < log_next_seq) {
+ while (seq < log_next_seq) {
struct printk_log *msg = log_from_idx(idx);

- len -= msg_print_text(msg, prev, true, NULL, 0);
- prev = msg->flags;
idx = log_next(idx);
seq++;
+
+ if (len - msg_print_text(msg, 0, true, NULL, 0) <= size)
+ break;
+
+ len -= msg_print_text(msg, prev, true, NULL, 0);
+ prev = msg->flags;
}

/* last message fitting into this dump */
@@ -2774,12 +2778,16 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
seq = dumper->cur_seq;
idx = dumper->cur_idx;
prev = 0;
- while (l > size && seq < dumper->next_seq) {
+ while (seq < dumper->next_seq) {
struct printk_log *msg = log_from_idx(idx);

- l -= msg_print_text(msg, prev, true, NULL, 0);
idx = log_next(idx);
seq++;
+
+ if (l - msg_print_text(msg, 0, true, NULL, 0) <= size)
+ break;
+
+ l -= msg_print_text(msg, prev, true, NULL, 0);
prev = msg->flags;
}

--
1.8.3.4


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