tpm: Make the TPM character devices non-seekable

The TPM character devices expose a sequential command/response
interface, but their open handlers leave FMODE_PREAD and FMODE_PWRITE
enabled.

After a command leaves a response pending, pread(fd, buf, 16, 0x1400)
passes 0x1400 as *off to tpm_common_read(). The transfer length is
bounded by response_length, but the offset is used unchecked when
forming data_buffer + *off. A sufficiently large offset therefore causes
an out-of-bounds heap read through copy_to_user() and, if the copy
succeeds, an out-of-bounds zero-write through the following memset().

Positional I/O does not provide coherent semantics for this interface.
An arbitrary pread offset cannot represent how much of a response has
been consumed sequentially. The write callback always stores a command
at the start of data_buffer, while pwrite() does not update file->f_pos
and can leave the sequential read cursor stale.

Call nonseekable_open() from both open handlers. This removes
FMODE_PREAD and FMODE_PWRITE, causing positional reads and writes to
fail with -ESPIPE before reaching the TPM callbacks, and explicitly
marks the files non-seekable. Normal read() and write() continue to use
the existing sequential f_pos cursor, leaving the response state machine
unchanged.

Tested on Linux 6.12 with KASAN and a swtpm TPM2 device:

 - sequential partial reads returned the complete response
 - pread() and preadv() with offset 0x1400 returned -ESPIPE
 - pwrite() and pwritev() with offset zero returned -ESPIPE
 - the pending response remained intact after the rejected operations
 - a subsequent normal command/response cycle completed normally
 - no KASAN report was produced.

Fixes: 9488585b21 ("tpm: add support for partial reads")
Link: https://lore.kernel.org/all/20260710090217.191289-1-yong010301@gmail.com/
Cc: stable@vger.kernel.org
Signed-off-by: Jaewon Yang <yong010301@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/tpm/tpm-dev.c

@@ -36,7 +36,7 @@ static int tpm_open(struct inode *inode, struct file *file)
 
 	tpm_common_open(file, chip, priv, NULL);
 
-	return 0;
+	return nonseekable_open(inode, file);
 
  out:
 	clear_bit(0, &chip->is_open);

drivers/char/tpm/tpmrm-dev.c

@@ -29,7 +29,7 @@ static int tpmrm_open(struct inode *inode, struct file *file)
 
 	tpm_common_open(file, chip, &priv->priv, &priv->space);
 
-	return 0;
+	return nonseekable_open(inode, file);
 }
 
 static int tpmrm_release(struct inode *inode, struct file *file)