[PATCH printk] printk_ringbuffer: Fix get_data() size sanity check

[John Ogness]

Commit cc3bad11de6e ("printk_ringbuffer: Fix check of valid data
size when blk_lpos overflows") added sanity checking to get_data()
to avoid returning data of illegal sizes (too large or too small).
It uses the helper function data_check_size() for the check.
However, data_check_size() expects the size of the data, not the
size of the data block. get_data() is providing the size of the
data block.  This means that if the data size (text_buf_size) is
the maximum legal size:

sizeof(prb_data_block) + text_buf_size == DATA_SIZE(data_ring) / 2

data_check_size() will report failure because it adds
sizeof(prb_data_block) to the provided size. The sanity check in
get_data() is counting the data block header twice. The result is
that the reader fails to read the legal record.

Since get_data() subtracts the data block header size before returning,
move the sanity check to after the subtraction.

Luckily printk() is not vulnerable to this problem because
truncate_msg() limits printk-messages to 1/4 of the ringbuffer.
Indeed, by adjusting the printk_ringbuffer KUnit test, which does not
use printk() and its truncate_msg() check, it is easy to see that the
reader fails and the WARN_ON is triggered.

Fixes: cc3bad11de6e ("printk_ringbuffer: Fix check of valid data size when blk_lpos overflows")
Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
---
 kernel/printk/printk_ringbuffer.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 56c8e3d031f49..613a6524cdd81 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -1302,10 +1302,6 @@ static const char *get_data(struct prb_data_ring *data_ring,
 		return NULL;
 	}
 
-	/* Sanity check. Data-less blocks were handled earlier. */
-	if (WARN_ON_ONCE(!data_check_size(data_ring, *data_size) || !*data_size))
-		return NULL;
-
 	/* A valid data block will always be aligned to the ID size. */
 	if (WARN_ON_ONCE(blk_lpos->begin != ALIGN(blk_lpos->begin, sizeof(db->id))) ||
 	    WARN_ON_ONCE(blk_lpos->next != ALIGN(blk_lpos->next, sizeof(db->id)))) {
@@ -1319,6 +1315,10 @@ static const char *get_data(struct prb_data_ring *data_ring,
 	/* Subtract block ID space from size to reflect data size. */
 	*data_size -= sizeof(db->id);
 
+	/* Sanity check. Data-less blocks were handled earlier. */
+	if (WARN_ON_ONCE(!data_check_size(data_ring, *data_size) || !*data_size))
+		return NULL;
+
 	return &db->data[0];
 }
 

base-commit: 9095f233c0258e9a05e958c7d822eb38681e7a5a
-- 
2.47.3