iPXE will currently drop to TPL_APPLICATION whenever the current
system time is obtained via currticks(), since the system time
mechanism relies on a timer that can fire only when the TPL is below
TPL_CALLBACK.
This can cause unexpected behaviour if the system time is obtained in
the middle of an API call into iPXE by external code. For example,
MnpDxe sets up a 10ms periodic timer running at TPL_CALLBACK to poll
the underling EFI_SIMPLE_NETWORK_PROTOCOL device for received packets.
If the resulting poll within iPXE happens to hit a code path that
requires obtaining the current system time (e.g. due to reception of
an STP packet, which affects iPXE's blocked link timer), then iPXE
will end up temporarily dropping to TPL_APPLICATION. This can
potentially result in retriggering the MnpDxe periodic timer, causing
code to be unexpectedly re-entered.
Fix by recording the external TPL at any entry point into iPXE and
dropping only as far as this external TPL, rather than dropping
unconditionally to TPL_APPLICATION.
The side effect of this change is that iPXE's view of the current
system time will be frozen for the duration of any API calls made into
iPXE by external code at TPL_CALLBACK or above. Since any such
external code is already responsible for allowing execution at
TPL_APPLICATION to occur, then this should not cause a problem in
practice.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
The EFI_RNG_PROTOCOL on the Microsoft Surface Go does not generate
random numbers. Successive calls to GetRNG() without any intervening
I/O operations (such as writing to the console) will produce identical
results. Successive reboots will produce identical results.
It is unclear what the Microsoft Surface Go is attempting to use as an
entropy source, but it is demonstrably producing zero bits of entropy.
The failure is already detected by the ANS-mandated Repetition Count
Test performed as part of our GetEntropy implementation. This
currently results in the entropy source being marked as broken, with
the result that iPXE refuses to perform any operations that require a
working entropy source.
We cannot use the existing EFI driver blacklisting mechanism to unload
the broken driver, since the RngDxe driver is integrated into the
DxeCore image.
Work around the broken driver by checking for consecutive identical
results returned by EFI_RNG_PROTOCOL and falling back to the original
timer-based entropy source.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Commit c89a446 ("[efi] Run at TPL_CALLBACK to protect against UEFI
timers") introduced a regression in the EFI entropy gathering code.
When the EFI_RNG_PROTOCOL is not present, we fall back to using timer
interrupts (as for the BIOS build). Since timer interrupts are
disabled at TPL_CALLBACK, WaitForEvent() fails and no entropy can be
gathered.
Fix by dropping to TPL_APPLICATION while entropy gathering is enabled.
Reported-by: Andreas Hammarskjöld <junior@2PintSoftware.com>
Tested-by: Andreas Hammarskjöld <junior@2PintSoftware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Michael Brown