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Subject[PATCH next v2 1/2] printk: avoid and/or handle record truncation
Date
If a reader provides a buffer that is smaller than the message text,
the @text_len field of @info will have a value larger than the buffer
size. If readers blindly read @text_len bytes of data without
checking the size, they will read beyond their buffer.

Add this check to record_print_text() to properly recognize when such
truncation has occurred.

Add a maximum size argument to the ringbuffer function to extend
records so that records can not be created that are larger than the
buffer size of readers.

When extending records (LOG_CONT), do not extend records beyond
LOG_LINE_MAX since that is the maximum size available in the buffers
used by consoles and syslog.

Fixes: f5f022e53b87 ("printk: reimplement log_cont using record extension")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
---
kernel/printk/printk.c | 9 ++++++++-
kernel/printk/printk_ringbuffer.c | 12 ++++++++++--
kernel/printk/printk_ringbuffer.h | 2 +-
3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 78f68b4830dc..04cf1e00478f 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1357,6 +1357,13 @@ static size_t record_print_text(struct printk_record *r, bool syslog,
size_t len = 0;
char *next;

+ /*
+ * If the message was truncated because the buffer was not large
+ * enough, treat the available text as if it were the full text.
+ */
+ if (text_len > buf_size)
+ text_len = buf_size;
+
prefix_len = info_print_prefix(r->info, syslog, time, prefix);

/*
@@ -1911,7 +1918,7 @@ static size_t log_output(int facility, int level, enum log_flags lflags,
struct printk_record r;

prb_rec_init_wr(&r, text_len);
- if (prb_reserve_in_last(&e, prb, &r, caller_id)) {
+ if (prb_reserve_in_last(&e, prb, &r, caller_id, LOG_LINE_MAX)) {
memcpy(&r.text_buf[r.info->text_len], text, text_len);
r.info->text_len += text_len;
if (lflags & LOG_NEWLINE) {
diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 13b94b92342e..2493348a1631 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -202,7 +202,8 @@
* // specify additional 5 bytes text space to extend
* prb_rec_init_wr(&r, 5);
*
- * if (prb_reserve_in_last(&e, &test_rb, &r, printk_caller_id())) {
+ * // try to extend, but only if it does not exceed 32 bytes
+ * if (prb_reserve_in_last(&e, &test_rb, &r, printk_caller_id()), 32) {
* snprintf(&r.text_buf[r.info->text_len],
* r.text_buf_size - r.info->text_len, "hello");
*
@@ -1309,6 +1310,7 @@ static struct prb_desc *desc_reopen_last(struct prb_desc_ring *desc_ring,
* @rb: The ringbuffer to re-reserve and extend data in.
* @r: The record structure to allocate buffers for.
* @caller_id: The caller ID of the caller (reserving writer).
+ * @max_size: Fail if the extended size would be greater than this.
*
* This is the public function available to writers to re-reserve and extend
* data.
@@ -1343,7 +1345,7 @@ static struct prb_desc *desc_reopen_last(struct prb_desc_ring *desc_ring,
* @r->info->text_len after concatenating.
*/
bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
- struct printk_record *r, u32 caller_id)
+ struct printk_record *r, u32 caller_id, unsigned int max_size)
{
struct prb_desc_ring *desc_ring = &rb->desc_ring;
struct printk_info *info;
@@ -1389,6 +1391,9 @@ bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer
if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
goto fail;

+ if (r->text_buf_size > max_size)
+ goto fail;
+
r->text_buf = data_alloc(rb, &rb->text_data_ring, r->text_buf_size,
&d->text_blk_lpos, id);
} else {
@@ -1410,6 +1415,9 @@ bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer
if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
goto fail;

+ if (r->text_buf_size > max_size)
+ goto fail;
+
r->text_buf = data_realloc(rb, &rb->text_data_ring, r->text_buf_size,
&d->text_blk_lpos, id);
}
diff --git a/kernel/printk/printk_ringbuffer.h b/kernel/printk/printk_ringbuffer.h
index 0adaa685d1ca..5dc9d022db07 100644
--- a/kernel/printk/printk_ringbuffer.h
+++ b/kernel/printk/printk_ringbuffer.h
@@ -303,7 +303,7 @@ static inline void prb_rec_init_wr(struct printk_record *r,
bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
struct printk_record *r);
bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
- struct printk_record *r, u32 caller_id);
+ struct printk_record *r, u32 caller_id, unsigned int max_size);
void prb_commit(struct prb_reserved_entry *e);
void prb_final_commit(struct prb_reserved_entry *e);

--
2.20.1
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