On Tue, Sep 23, 2025 at 02:33:05PM +0100, Jonathan McDowell wrote: > From: Jonathan McDowell <[email protected]> > > The HW RNG core allows for manual selection of which RNG device to use, > but does not allow for no device to be enabled. It may be desirable to > do this on systems with only a single suitable hardware RNG, where we > need exclusive access to other functionality on this device. In > particular when performing TPM firmware upgrades this lets us ensure the > kernel does not try to access the device. > > Before: > > root@debian-qemu-efi:~# grep "" /sys/devices/virtual/misc/hw_random/rng_* > /sys/devices/virtual/misc/hw_random/rng_available:tpm-rng-0 > /sys/devices/virtual/misc/hw_random/rng_current:tpm-rng-0 > /sys/devices/virtual/misc/hw_random/rng_quality:1024 > /sys/devices/virtual/misc/hw_random/rng_selected:0 > > After: > > root@debian-qemu-efi:~# grep "" /sys/devices/virtual/misc/hw_random/rng_* > /sys/devices/virtual/misc/hw_random/rng_available:tpm-rng-0 none > /sys/devices/virtual/misc/hw_random/rng_current:tpm-rng-0 > /sys/devices/virtual/misc/hw_random/rng_quality:1024 > /sys/devices/virtual/misc/hw_random/rng_selected:0 > > root@debian-qemu-efi:~# echo none > > /sys/devices/virtual/misc/hw_random/rng_current > root@debian-qemu-efi:~# grep "" /sys/devices/virtual/misc/hw_random/rng_* > /sys/devices/virtual/misc/hw_random/rng_available:tpm-rng-0 none > /sys/devices/virtual/misc/hw_random/rng_current:none > grep: /sys/devices/virtual/misc/hw_random/rng_quality: No such device > /sys/devices/virtual/misc/hw_random/rng_selected:1 > > (Observe using bpftrace no calls to TPM being made) > > root@debian-qemu-efi:~# echo "" > > /sys/devices/virtual/misc/hw_random/rng_current > root@debian-qemu-efi:~# grep "" /sys/devices/virtual/misc/hw_random/rng_* > /sys/devices/virtual/misc/hw_random/rng_available:tpm-rng-0 none > /sys/devices/virtual/misc/hw_random/rng_current:tpm-rng-0 > /sys/devices/virtual/misc/hw_random/rng_quality:1024 > /sys/devices/virtual/misc/hw_random/rng_selected:0 > > (Observe using bpftrace that calls to the TPM resume) > > Signed-off-by: Jonathan McDowell <[email protected]> > --- > v3: Always set cur_rng_set_by_user if user forces no HW RNG. > v2: If the user manually forces the HWRNG to none do not override this > when a new driver is loaded. Pointed out by Herbert Xu. > drivers/char/hw_random/core.c | 9 ++++++--- > 1 file changed, 6 insertions(+), 3 deletions(-)
Patch applied. Thanks. -- Email: Herbert Xu <[email protected]> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
