Re: [PATCH] soc: fsl: qe: Replace one-element array and use struct_size() helper

Kees Cook
2020-05-18T22:56:55Z

On Mon, May 18, 2020 at 05:19:04PM -0500, Gustavo A. R. Silva wrote:
> The current codebase makes use of one-element arrays in the following
> form:
>
> struct something {
> int length;
> u8 data[1];
> };
>
> struct something *instance;
>
> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
> instance->length = size;
> memcpy(instance->data, source, size);
>
> but the preferred mechanism to declare variable-length types such as
> these ones is a flexible array member[1][2], introduced in C99:
>
> struct foo {
> int stuff;
> struct boo array[];
> };
>
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on. So, replace
> the one-element array with a flexible-array member.
>
> Also, make use of the new struct_size() helper to properly calculate the
> size of struct qe_firmware.
>
> This issue was found with the help of Coccinelle and, audited and fixed
> _manually_.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
> drivers/soc/fsl/qe/qe.c | 4 ++--
> include/soc/fsl/qe/qe.h | 2 +-
> 2 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/soc/fsl/qe/qe.c b/drivers/soc/fsl/qe/qe.c
> index 447146861c2c1..2df20d6f85fa4 100644
> --- a/drivers/soc/fsl/qe/qe.c
> +++ b/drivers/soc/fsl/qe/qe.c
> @@ -448,7 +448,7 @@ int qe_upload_firmware(const struct qe_firmware *firmware)
> unsigned int i;
> unsigned int j;
> u32 crc;
> - size_t calc_size = sizeof(struct qe_firmware);
> + size_t calc_size;
> size_t length;
> const struct qe_header *hdr;
>
> @@ -480,7 +480,7 @@ int qe_upload_firmware(const struct qe_firmware *firmware)
> }
>
> /* Validate the length and check if there's a CRC */
> - calc_size += (firmware->count - 1) * sizeof(struct qe_microcode);
> + calc_size = struct_size(firmware, microcode, firmware->count);
>
> for (i = 0; i < firmware->count; i++)
> /*
> diff --git a/include/soc/fsl/qe/qe.h b/include/soc/fsl/qe/qe.h
> index e282ac01ec081..3feddfec9f87d 100644
> --- a/include/soc/fsl/qe/qe.h
> +++ b/include/soc/fsl/qe/qe.h
> @@ -307,7 +307,7 @@ struct qe_firmware {
> u8 revision; /* The microcode version revision */
> u8 padding; /* Reserved, for alignment */
> u8 reserved[4]; /* Reserved, for future expansion */
> - } __attribute__ ((packed)) microcode[1];
> + } __packed microcode[];
> /* All microcode binaries should be located here */
> /* CRC32 should be located here, after the microcode binaries */
> } __attribute__ ((packed));
> --
> 2.26.2
>

Hm, looking at this code, I see a few other things that need to be
fixed:

1) drivers/tty/serial/ucc_uart.c does not do a be32_to_cpu() conversion
on the length test (understandably, a little-endian system has never run
this code since it's ppc specific), but it's still wrong:

if (firmware->header.length != fw->size) {

compare to the firmware loader:

length = be32_to_cpu(hdr->length);

2) drivers/soc/fsl/qe/qe.c does not perform bounds checking on the
per-microcode offsets, so the uploader might send data outside the
firmware buffer. Perhaps:

diff --git a/drivers/soc/fsl/qe/qe.c b/drivers/soc/fsl/qe/qe.c
index 447146861c2c..c4e0bc452f03 100644
--- a/drivers/soc/fsl/qe/qe.c
+++ b/drivers/soc/fsl/qe/qe.c
@@ -451,6 +451,7 @@ int qe_upload_firmware(const struct qe_firmware *firmware)
size_t calc_size = sizeof(struct qe_firmware);
size_t length;
const struct qe_header *hdr;
+ void *firmware_end;

if (!firmware) {
printk(KERN_ERR "qe-firmware: invalid pointer\n");
@@ -491,19 +492,39 @@ int qe_upload_firmware(const struct qe_firmware *firmware)
calc_size += sizeof(__be32) *
be32_to_cpu(firmware->microcode[i].count);

- /* Validate the length */
+ /* Validate total length */
if (length != calc_size + sizeof(__be32)) {
printk(KERN_ERR "qe-firmware: invalid length\n");
return -EPERM;
}

/* Validate the CRC */
- crc = be32_to_cpu(*(__be32 *)((void *)firmware + calc_size));
+ firmware_end = (void *)firmware + calc_size;
+ crc = be32_to_cpu(*(__be32 *)firmware_end);
if (crc != crc32(0, firmware, calc_size)) {
printk(KERN_ERR "qe-firmware: firmware CRC is invalid\n");
return -EIO;
}

+ /* Validate ucode lengths and offsets */
+ for (i = 0; i < firmware->count; i++) {
+ const struct qe_microcode *ucode = &firmware->microcode[i];
+ __be32 *code;
+ size_t count;
+
+ if (!ucode->code_offset)
+ continue;
+
+ code = (void *)firmware + be32_to_cpu(ucode->code_offset);
+ count = be32_to_cpu(ucode->count) * sizeof(*code);
+
+ if (code < firmware || code >= firmware_end ||
+ code + count < firmware || code + count >= firmware_end) {
+ printk(KERN_ERR "qe-firmware: invalid ucode offset\n");
+ return -EIO;
+ }
+ }
+
/*
* If the microcode calls for it, split the I-RAM.
*/

I haven't tested this.

--
Kees Cook