[edk2-devel] [PATCH v2 RESEND 0/2] security fix: unlimited FV recursion, round 2 (DXE Core) -- push #1137
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November 21, 2020 02:33
…ode() FindChildNode() has two callers: GetSection(), and FindChildNode() itself. - At the GetSection() call site, a positive (i.e., nonzero) SectionInstance is passed. This is because GetSection() takes a zero-based (UINTN) SectionInstance, and then passes Instance=(SectionInstance+1) to FindChildNode(). - For reaching the recursive FindChildNode() call site, a section type mismatch, or a section instance mismatch, is necessary. This means, respectively, that SectionInstance will either not have been decreased, or not to zero anyway, at the recursive FindChildNode() call site. Add two ASSERT()s to FindChildNode(), for expressing the (SectionSize>0) invariant. In turn, the invariant provides the explanation why, after the recursive call, a zero SectionInstance implies success. Capture it in a comment. Cc: Dandan Bi <[email protected]> Cc: Hao A Wu <[email protected]> Cc: Jian J Wang <[email protected]> Cc: Liming Gao <[email protected]> Cc: Philippe Mathieu-Daudé <[email protected]> Signed-off-by: Laszlo Ersek <[email protected]> Reviewed-by: Liming Gao <[email protected]> Reviewed-by: Philippe Mathieu-Daudé <[email protected]> Message-Id: <[email protected]>
The DXE Core sets up a protocol notify function in its entry point, for
instances of the Firmware Volume Block2 Protocol:
DxeMain() [DxeMain/DxeMain.c]
FwVolDriverInit() [FwVol/FwVol.c]
Assume that a 3rd party UEFI driver or application installs an FVB
instance, with crafted contents. The notification function runs:
NotifyFwVolBlock() [FwVol/FwVol.c]
installing an instance of the Firmware Volume 2 Protocol on the handle.
(Alternatively, assume that a 3rd party application calls
gDS->ProcessFirmwareVolume(), which may also produce a Firmware Volume 2
Protocol instance.)
The EFI_FIRMWARE_VOLUME2_PROTOCOL.ReadSection() member performs "a
depth-first, left-to-right search algorithm through all sections found in
the specified file" (quoting the PI spec), as follows:
FvReadFileSection() [FwVol/FwVolRead.c]
GetSection() [SectionExtraction/CoreSectionExtraction.c]
FindChildNode() [SectionExtraction/CoreSectionExtraction.c]
FindChildNode() // recursive call
FindChildNode() is called recursively for encapsulation sections.
Currently this recursion is not limited. Introduce a new PCD
(fixed-at-build, or patchable-in-module), and make FindChildNode() track
the section nesting depth against that PCD.
Cc: Dandan Bi <[email protected]>
Cc: Hao A Wu <[email protected]>
Cc: Jian J Wang <[email protected]>
Cc: Liming Gao <[email protected]>
Cc: Philippe Mathieu-Daudé <[email protected]>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1743
Signed-off-by: Laszlo Ersek <[email protected]>
Reviewed-by: Liming Gao <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <[email protected]>
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https://www.redhat.com/archives/edk2-devel-archive/2020-November/msg00865.html