media: ngene: Fix out-of-bounds bug in ngene_command_config_free_buf()

Fix an 11-year old bug in ngene_command_config_free_buf() while
addressing the following warnings caught with -Warray-bounds:

arch/alpha/include/asm/string.h:22:16: warning: '__builtin_memcpy' offset [12, 16] from the object at 'com' is out of the bounds of referenced subobject 'config' with type 'unsigned char' at offset 10 [-Warray-bounds]
arch/x86/include/asm/string_32.h:182:25: warning: '__builtin_memcpy' offset [12, 16] from the object at 'com' is out of the bounds of referenced subobject 'config' with type 'unsigned char' at offset 10 [-Warray-bounds]

The problem is that the original code is trying to copy 6 bytes of
data into a one-byte size member _config_ of the wrong structue
FW_CONFIGURE_BUFFERS, in a single call to memcpy(). This causes a
legitimate compiler warning because memcpy() overruns the length
of &com.cmd.ConfigureBuffers.config. It seems that the right
structure is FW_CONFIGURE_FREE_BUFFERS, instead, because it contains
6 more members apart from the header _hdr_. Also, the name of
the function ngene_command_config_free_buf() suggests that the actual
intention is to ConfigureFreeBuffers, instead of ConfigureBuffers
(which takes place in the function ngene_command_config_buf(), above).

Fix this by enclosing those 6 members of struct FW_CONFIGURE_FREE_BUFFERS
into new struct config, and use &com.cmd.ConfigureFreeBuffers.config as
the destination address, instead of &com.cmd.ConfigureBuffers.config,
when calling memcpy().

This also helps with the ongoing efforts to globally enable
-Warray-bounds and get us closer to being able to tighten the
FORTIFY_SOURCE routines on memcpy().

Link: KSPP/linux#109
Fixes: dae52d0 ("V4L/DVB: ngene: Initial check-in")
Cc: [email protected]
Reported-by: kernel test robot <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Signed-off-by: Gustavo A. R. Silva <[email protected]>
Link: https://lore.kernel.org/linux-hardening/20210420001631.GA45456@embeddedor/

drivers/media/pci/ngene/ngene-core.c

@@ -404,7 +404,7 @@ static int ngene_command_config_free_buf(struct ngene *dev, u8 *config)
 
 	com.cmd.hdr.Opcode = CMD_CONFIGURE_FREE_BUFFER;
 	com.cmd.hdr.Length = 6;
-	memcpy(&com.cmd.ConfigureBuffers.config, config, 6);
+	memcpy(&com.cmd.ConfigureFreeBuffers.config, config, 6);
 	com.in_len = 6;
 	com.out_len = 0;
 

drivers/media/pci/ngene/ngene.h

@@ -407,12 +407,14 @@ enum _BUFFER_CONFIGS {
 
 struct FW_CONFIGURE_FREE_BUFFERS {
 	struct FW_HEADER hdr;
-	u8   UVI1_BufferLength;
-	u8   UVI2_BufferLength;
-	u8   TVO_BufferLength;
-	u8   AUD1_BufferLength;
-	u8   AUD2_BufferLength;
-	u8   TVA_BufferLength;
+	struct {
+		u8   UVI1_BufferLength;
+		u8   UVI2_BufferLength;
+		u8   TVO_BufferLength;
+		u8   AUD1_BufferLength;
+		u8   AUD2_BufferLength;
+		u8   TVA_BufferLength;
+	} __packed config;
 } __attribute__ ((__packed__));
 
 struct FW_CONFIGURE_UART {