[PATCH v3 0/4] pm: Ensure exclusive userspace access when using /dev/tpm

Jonathan McDowell
2025-10-20T11:43:06Z

I hit a problem where ~ 1% of TPM firmware upgrades were failing.
Investigating revealed the issue was that although the upgrade tool uses
/dev/tpm0 this does not actually prevent access via /dev/tpmrm0, nor
internal kernel users. It *does* prevent access to others via /dev/tpm0

So the upgrade process started, the HW RNG came in to get some
randomness in the middle, did the HMAC context dance, and confused
everything to the point the TPM was no longer visible to the OS even
after a reboot.

Thankfully I've been able to recover those devices, but really what I'd
like is the ability for a userspace tool to exclusively access the TPM
without something coming in behind it. Given the lightweight attempt at
locking that already exists I think this was the original intention.

I've reworked this series based on feedback received.

Firstly, it's been reordered TPM sharing functionality doesn't break
during bisection.

Secondly, the O_EXCL check has been tightend up to ensure the caller is
also opening the device O_RDWR. Callers shouldn't really be opening the
TPM except for reading + writing, but this should help guard against
unexpected flags usage a bit more.

Finally, this revision keeps the prohibition on more than one user of
/dev/tpm#, to avoid unexpected breakages for clients that expect this to
guard against multiple invocations. A client only then needs to use
O_EXCL if it wants to prevent *all* other access, even with
ContextSaves, such as the firmware upgrade case.

(I've sent a separate standalone patch that allows the TPM HW RNG to be
disabled at run time, and it's now in -next, but even with that I think
something like this is a good idea as well.)

Jonathan McDowell (4):
tpm: Remove tpm_find_get_ops
tpm: Add O_EXCL for exclusive /dev/tpm access
tpm: Include /dev/tpmrm<n> when checking exclusive userspace TPM
access
tpm: Allow for exclusive TPM access when using /dev/tpm<n>

drivers/char/tpm/tpm-chip.c | 90 +++++++++++++++----------------
drivers/char/tpm/tpm-dev-common.c | 8 +--
drivers/char/tpm/tpm-dev.c | 35 ++++++++++--
drivers/char/tpm/tpm-dev.h | 1 +
drivers/char/tpm/tpm-interface.c | 20 +++++--
drivers/char/tpm/tpm.h | 3 +-
drivers/char/tpm/tpm2-space.c | 5 +-
drivers/char/tpm/tpm_tis_core.c | 3 +-
drivers/char/tpm/tpmrm-dev.c | 20 ++++++-
include/linux/tpm.h | 4 +-
10 files changed, 124 insertions(+), 65 deletions(-)

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2.51.0