Re: [PATCH next 1/2] printk: avoid and/or handle record truncation

[Marek Szyprowski]

Hi John,

On 26.09.2020 03:55, John Ogness wrote:
> 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 needs to occur.
>
> 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@xxxxxxxxxxxxx>
> Reported-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>

Thanks for fixing this issue!

Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>

> ---
>   kernel/printk/printk.c            |  7 ++++++-
>   kernel/printk/printk_ringbuffer.c | 12 ++++++++++--
>   kernel/printk/printk_ringbuffer.h |  2 +-
>   3 files changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 78f68b4830dc..270f19b60e6f 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1357,6 +1357,11 @@ static size_t record_print_text(struct printk_record *r, bool syslog,
>   	size_t len = 0;
>   	char *next;
>   
> +	if (text_len > buf_size) {
> +		text_len = buf_size;
> +		truncated = true;
> +	}
> +
>   	prefix_len = info_print_prefix(r->info, syslog, time, prefix);
>   
>   	/*
> @@ -1911,7 +1916,7 @@ 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);
>   

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland