linux-kernel - [PATCH v2 RESEND] dt-bindings: chosen: document kaslr-seed property

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Message-ID: <20170717220511.GA140556@beast>
Date:   Mon, 17 Jul 2017 15:05:11 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Frank Rowand <frank.rowand@...y.com>,
        James Morse <james.morse@....com>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 RESEND] dt-bindings: chosen: document kaslr-seed property

Document the /chosen/kaslr-seed property (and its interaction with the
EFI_RNG_PROTOCOL API). Thanks to Ard for clarifications.

Signed-off-by: Kees Cook <keescook@...omium.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Acked-by: Mark Rutland <mark.rutland@....com>
Acked-by: Will Deacon <will.deacon@....com>
---
v2:
- improved the language a bit based on Ard's notes
- added Acks.
- (resent to include lists)
---
 Documentation/devicetree/bindings/chosen.txt | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt
index dee3f5d9df26..e3b13ea7d2ae 100644
--- a/Documentation/devicetree/bindings/chosen.txt
+++ b/Documentation/devicetree/bindings/chosen.txt
@@ -5,9 +5,31 @@ The chosen node does not represent a real device, but serves as a place
 for passing data between firmware and the operating system, like boot
 arguments. Data in the chosen node does not represent the hardware.
 
+The following properties are recognized:
 
-stdout-path property
---------------------
+
+kaslr-seed
+-----------
+
+This property is used when booting with CONFIG_RANDOMIZE_BASE as the
+entropy used to randomize the kernel image base address location. Since
+it is used directly, this value is intended only for KASLR, and should
+not be used for other purposes (as it may leak information about KASLR
+offsets). It is parsed as a u64 value, e.g.
+
+/ {
+	chosen {
+		kaslr-seed = <0xfeedbeef 0xc0def00d>;
+	};
+};
+
+Note that if this property is set from UEFI (or a bootloader in EFI
+mode) when EFI_RNG_PROTOCOL is supported, it will be overwritten by
+the Linux EFI stub (which will populate the property itself, using
+EFI_RNG_PROTOCOL).
+
+stdout-path
+-----------
 
 Device trees may specify the device to be used for boot console output
 with a stdout-path property under /chosen, as described in the Devicetree
-- 
2.7.4


-- 
Kees Cook
Pixel Security