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

[Jarkko Sakkinen]

oN Sat, Jul 18, 2026 at 09:13:30PM +0300, Jarkko Sakkinen wrote:
> 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@xxxxxxxxx/
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Jaewon Yang <yong010301@xxxxxxxxx>
> > ---
> > 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@xxxxxxxxxx>

[A slow response time due on holiday until end of this Month.]

BR, Jarkko