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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>
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As per commit c89a446 ("[efi] Run at TPL_CALLBACK to protect against
UEFI timers") we expect to run at TPL_CALLBACK almost all of the time.
Various code paths rely on this assumption. Code paths that need to
temporarily lower the TPL (e.g. for entropy gathering) will restore it
to TPL_CALLBACK.
The entropy gathering code will be run during DRBG initialisation,
which happens during the call to startup(). In the case of iPXE
compiled as an EFI application this code will run within the scope of
efi_snp_claim() and so will execute at TPL_CALLBACK as expected.
In the case of iPXE compiled as an EFI driver the code will
incorrectly run at TPL_APPLICATION since there is nothing within the
EFI driver entry point that raises (and restores) the TPL. The net
effect is that a build that includes the entropy-gathering code
(e.g. a build with HTTPS enabled) will return from the driver entry
point at TPL_CALLBACK, which causes a system lockup.
Fix by raising and restoring the TPL within the EFI driver entry
point.
Debugged-by: Ignat Korchagin <ignat@cloudflare.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
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Enable -fstack-protector for EFI builds, where binary size is less
critical than for BIOS builds.
The stack cookie must be constructed immediately on entry, which
prohibits the use of any viable entropy source. Construct a cookie by
XORing together various mildly random quantities to produce a value
that will at least not be identical on each run.
On detecting a stack corruption, attempt to call Exit() with an
appropriate error. If that fails, then lock up the machine since
there is no other safe action that can be taken.
The old conditional check for support of -fno-stack-protector is
omitted since this flag dates back to GCC 4.1.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
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Signed-off-by: Michael Brown <mcb30@ipxe.org>
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