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

From: Kees Cook <[email-masked]>
Date: Mon, 18 May 2020 15:56:51 -0700

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 <[email-masked]>
> ---
>  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

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