wifi: brcmfmac: pcie: handle randbuf allocation failure

The kzalloc() in brcmf_pcie_download_fw_nvram() will return null
if the physical memory has run out. As a result, if we use
get_random_bytes() to generate random bytes in the randbuf, the
null pointer dereference bug will happen.

In order to prevent allocation failure, this patch adds a separate
function using buffer on kernel stack to generate random bytes in
the randbuf, which could prevent the kernel stack from overflow.

Fixes: 91918ce88d9f ("wifi: brcmfmac: pcie: Provide a buffer of random bytes to the device")
Suggested-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Duoming Zhou <[e-mail]>
Signed-off-by: Kalle Valo <[e-mail]>
Link: https://msgid.link/[e-mail]

drivers/net/wireless/broadcom/brcm80211/brcmfmac/pcie.c

···
 #define BRCMF_RANDOM_SEED_MAGIC		0xfeedc0de
 #define BRCMF_RANDOM_SEED_LENGTH	0x100
 
+static noinline_for_stack void
+brcmf_pcie_provide_random_bytes(struct brcmf_pciedev_info *devinfo, u32 address)
+{
+	u8 randbuf[BRCMF_RANDOM_SEED_LENGTH];
+
+	get_random_bytes(randbuf, BRCMF_RANDOM_SEED_LENGTH);
+	memcpy_toio(devinfo->tcm + address, randbuf, BRCMF_RANDOM_SEED_LENGTH);
+}
+
 static int brcmf_pcie_download_fw_nvram(struct brcmf_pciedev_info *devinfo,
 					const struct firmware *fw, void *nvram,
 					u32 nvram_len)
···
 				.length = cpu_to_le32(rand_len),
 				.magic = cpu_to_le32(BRCMF_RANDOM_SEED_MAGIC),
 			};
-			void *randbuf;
 
 			/* Some Apple chips/firmwares expect a buffer of random
 			 * data to be present before NVRAM
···
 				    sizeof(footer));
 
 			address -= rand_len;
-			randbuf = kzalloc(rand_len, GFP_KERNEL);
-			get_random_bytes(randbuf, rand_len);
-			memcpy_toio(devinfo->tcm + address, randbuf, rand_len);
-			kfree(randbuf);
+			brcmf_pcie_provide_random_bytes(devinfo, address);
 		}
 	} else {
 		brcmf_dbg(PCIE, "No matching NVRAM file found %s\n",