platform/x86/amd/pmc: Use flex array when calling amd_pmc_stb_debugfs_open_v2()

Currently in amd_pmc_stb_debugfs_open_v2() the buffer size is assumed to
be fixed and a second call to amd_pmc_stb_debugfs_open_v2() may race with
a process holding open another fd. This could change "fsize" to a
bigger size causing an out of bounds read.

Instead create a struct with a flexarray to solve this.

Suggested-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sanket Goswami <Sanket.Goswami@amd.com>
Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Link: https://lore.kernel.org/r/20231010145003.139932-1-Shyam-sundar.S-k@amd.com
[ij: renamed flex_arr -> stb_data_arr]
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

Author: Shyam Sundar S K

drivers/platform/x86/amd/pmc/pmc.c

@@ -52,7 +52,7 @@
 #define AMD_S2D_REGISTER_ARGUMENT	0xA88
 
 /* STB Spill to DRAM Parameters */
-#define S2D_TELEMETRY_BYTES_MAX		0x100000
+#define S2D_TELEMETRY_BYTES_MAX		0x100000U
 #define S2D_TELEMETRY_DRAMBYTES_MAX	0x1000000
 
 /* STB Spill to DRAM Message Definition */
@@ -122,6 +122,11 @@ enum s2d_arg {
 	S2D_DRAM_SIZE,
 };
 
+struct amd_pmc_stb_v2_data {
+	size_t size;
+	u8 data[] __counted_by(size);
+};
+
 struct amd_pmc_bit_map {
 	const char *name;
 	u32 bit_mask;
@@ -239,18 +244,15 @@ static const struct file_operations amd_pmc_stb_debugfs_fops = {
 static int amd_pmc_stb_debugfs_open_v2(struct inode *inode, struct file *filp)
 {
 	struct amd_pmc_dev *dev = filp->f_inode->i_private;
-	u32 *buf, fsize, num_samples, val, stb_rdptr_offset = 0;
+	u32 fsize, num_samples, val, stb_rdptr_offset = 0;
+	struct amd_pmc_stb_v2_data *stb_data_arr;
 	int ret;
 
 	/* Write dummy postcode while reading the STB buffer */
 	ret = amd_pmc_write_stb(dev, AMD_PMC_STB_DUMMY_PC);
 	if (ret)
 		dev_err(dev->dev, "error writing to STB: %d\n", ret);
 
-	buf = kzalloc(S2D_TELEMETRY_BYTES_MAX, GFP_KERNEL);
-	if (!buf)
-		return -ENOMEM;
-
 	/* Spill to DRAM num_samples uses separate SMU message port */
 	dev->msg_port = 1;
 
@@ -264,10 +266,16 @@ static int amd_pmc_stb_debugfs_open_v2(struct inode *inode, struct file *filp)
 	dev->msg_port = 0;
 	if (ret) {
 		dev_err(dev->dev, "error: S2D_NUM_SAMPLES not supported : %d\n", ret);
-		kfree(buf);
 		return ret;
 	}
 
+	fsize = min(num_samples, S2D_TELEMETRY_BYTES_MAX);
+	stb_data_arr = kmalloc(struct_size(stb_data_arr, data, fsize), GFP_KERNEL);
+	if (!stb_data_arr)
+		return -ENOMEM;
+
+	stb_data_arr->size = fsize;
+
 	/* Start capturing data from the last push location */
 	if (num_samples > S2D_TELEMETRY_BYTES_MAX) {
 		fsize  = S2D_TELEMETRY_BYTES_MAX;
@@ -277,20 +285,18 @@ static int amd_pmc_stb_debugfs_open_v2(struct inode *inode, struct file *filp)
 		stb_rdptr_offset = 0;
 	}
 
-	memcpy_fromio(buf, dev->stb_virt_addr + stb_rdptr_offset, fsize);
-	filp->private_data = buf;
+	memcpy_fromio(stb_data_arr->data, dev->stb_virt_addr + stb_rdptr_offset, fsize);
+	filp->private_data = stb_data_arr;
 
 	return 0;
 }
 
 static ssize_t amd_pmc_stb_debugfs_read_v2(struct file *filp, char __user *buf, size_t size,
 					   loff_t *pos)
 {
-	if (!filp->private_data)
-		return -EINVAL;
+	struct amd_pmc_stb_v2_data *data = filp->private_data;
 
-	return simple_read_from_buffer(buf, size, pos, filp->private_data,
-					S2D_TELEMETRY_BYTES_MAX);
+	return simple_read_from_buffer(buf, size, pos, data->data, data->size);
 }
 
 static int amd_pmc_stb_debugfs_release_v2(struct inode *inode, struct file *filp)