Re: [PATCH v2] tpm: Make the TPM character devices non-seekable

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Jul 13, 2026 at 02:11:47AM +0900, Jaewon Yang wrote:
> 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; and
> - no KASAN report was produced.
> 
> Fixes: 9488585b21be ("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>
> ---
> Changes in v2:
> - replace the response-buffer bounds check with nonseekable_open();
> - reject positional read and write at open time;
> - preserve the existing sequential read/write state machine.
> 
> The alternative response_length rework proposed during review was tested
> and not taken: a read-until-EOF loop hangs because cleanup resets *off
> without clearing response_length. It also treats an arbitrary positional
> offset as the consumption cursor; for example,
> 
>     pread(fd, &c, 1, 99)
> 
> on a 100-byte response can discard bytes 0 through 98 without returning
> them.
> 
>  drivers/char/tpm/tpm-dev.c   | 2 +-
>  drivers/char/tpm/tpmrm-dev.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm-dev.c b/drivers/char/tpm/tpm-dev.c
> index 2779a8738..74488f0a7 100644
> --- a/drivers/char/tpm/tpm-dev.c
> +++ b/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);
> diff --git a/drivers/char/tpm/tpmrm-dev.c b/drivers/char/tpm/tpmrm-dev.c
> index f48d4d9e1..19e8f2779 100644
> --- a/drivers/char/tpm/tpmrm-dev.c
> +++ b/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)
> -- 
> 2.43.0
> 

LGTM


Acked-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko