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SubjectRe: [PATCH next 1/2] printk: avoid and/or handle record truncation
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Date
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@linutronix.de>
> Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>

Thanks for fixing this issue!

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

> ---
> 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

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