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SubjectRe: [PATCH v2] cxl/mbox: Fix Payload Length check for Get Log command
On Wed, 4 Jan 2023 21:29:54 +0100
Robert Richter <rrichter@amd.com> wrote:

> Commit 2aeaf663b85e introduced strict checking for variable length
> payload size validation. The payload length of received data must
> match the size of the requested data by the caller except for the case
> where the min_out value is set.
>
> The Get Log command does not have a header with a length field set.
> The Log size is determined by the Get Supported Logs command (CXL 3.0,
> 8.2.9.5.1). However, the actual size can be smaller and the number of
> valid bytes in the payload output must be determined reading the
> Payload Length field (CXL 3.0, Table 8-36, Note 2).
>
> Two issues arise: The command can successfully complete with a payload
> length of zero. And, the valid payload length must then also be
> consumed by the caller.
>
> Change cxl_xfer_log() to pass the number of payload bytes back to the
> caller to determine the number of log entries. Implement the payload
> handling as a special case where mbox_cmd->size_out is consulted when
> cxl_internal_send_cmd() returns -EIO. A WARN_ONCE() is added to check
> that -EIO is only returned in case of an unexpected output size.
>
> Logs can be bigger than the maximum payload length and multiple Get
> Log commands can be issued. If the received payload size is smaller
> than the maximum payload size we can assume all valid bytes have been
> fetched. Stop sending further Get Log commands then.
>
> On that occasion, change debug messages to also report the opcodes of
> supported commands.
>
> The variable payload commands GET_LSA and SET_LSA could be also
> affected by this strict check, but SET_LSA cannot be broken because
> SET_LSA does not return an output payload, and GET_LSA never expects
> short reads.
>
> Fixes: 2aeaf663b85e ("cxl/mbox: Add variable output size validation for internal commands")
> Signed-off-by: Robert Richter <rrichter@amd.com>
Hi Robert, a few comments inline.

Thanks,

Jonathan

> ---
> drivers/cxl/core/mbox.c | 28 ++++++++++++++++++++--------
> 1 file changed, 20 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/cxl/core/mbox.c b/drivers/cxl/core/mbox.c
> index b03fba212799..e93df0d39022 100644
> --- a/drivers/cxl/core/mbox.c
> +++ b/drivers/cxl/core/mbox.c
> @@ -170,6 +170,8 @@ int cxl_internal_send_cmd(struct cxl_dev_state *cxlds,
> out_size = mbox_cmd->size_out;
> min_out = mbox_cmd->min_out;
> rc = cxlds->mbox_send(cxlds, mbox_cmd);
> + if (WARN_ONCE(rc == -EIO, "Bad return code: -EIO"))

This is unusual. Why the WARN_ONCE? How can an error code
be bad? It may well panic. Fine to have a dev_err() or similar
but this seems excessive.


> + return -ENXIO;
> if (rc)
> return rc;
>
> @@ -576,6 +578,17 @@ static int cxl_xfer_log(struct cxl_dev_state *cxlds, uuid_t *uuid, u32 size, u8
> };
>
> rc = cxl_internal_send_cmd(cxlds, &mbox_cmd);
> +
> + /*
> + * The output payload length that indicates the number
> + * of valid bytes can be smaller than the Log buffer
> + * size.
> + */
> + if (rc == -EIO && mbox_cmd.size_out < xfer_size) {
> + offset += mbox_cmd.size_out;
> + break;
> + }
> +
> if (rc < 0)
> return rc;
>
> @@ -584,7 +597,7 @@ static int cxl_xfer_log(struct cxl_dev_state *cxlds, uuid_t *uuid, u32 size, u8
> offset += xfer_size;
> }
>
> - return 0;
> + return offset;
> }
>
> /**
> @@ -608,13 +621,11 @@ static void cxl_walk_cel(struct cxl_dev_state *cxlds, size_t size, u8 *cel)
> u16 opcode = le16_to_cpu(cel_entry[i].opcode);
> struct cxl_mem_command *cmd = cxl_mem_find_command(opcode);
>
> - if (!cmd) {
> - dev_dbg(cxlds->dev,
> - "Opcode 0x%04x unsupported by driver", opcode);
> - continue;
> - }
> + if (cmd)
> + set_bit(cmd->info.id, cxlds->enabled_cmds);
>
> - set_bit(cmd->info.id, cxlds->enabled_cmds);
> + dev_dbg(cxlds->dev, "Opcode 0x%04x %ssupported by driver",
> + opcode, cmd ? "" : "un");

Unrelated change so doesn't belong in this patch. I'd also split the
dev_dbg into two paths both to reduce modification and because people
might grep for "unsupported by driver"


> }
> }
>
> @@ -695,11 +706,12 @@ int cxl_enumerate_cmds(struct cxl_dev_state *cxlds)
> }
>
> rc = cxl_xfer_log(cxlds, &uuid, size, log);
> - if (rc) {
> + if (rc < 0) {

Feels like passing in size as a pointer that is then updated might be cleaner.

> kvfree(log);
> goto out;
> }
>
> + size = (u32)rc;
> cxl_walk_cel(cxlds, size, log);
> kvfree(log);
>

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