[xen staging] libxl/arm: provide guests with random seed

[patchbot@xxxxxxx]

commit 33e48312cf28f6fb78c03b14514fa7718de97ef5
Author:     Sergiy Kibrik <Sergiy_Kibrik@xxxxxxxx>
AuthorDate: Tue Jul 6 06:33:45 2021 +0000
Commit:     Julien Grall <jgrall@xxxxxxxxxx>
CommitDate: Tue Jul 6 09:47:33 2021 +0100

    libxl/arm: provide guests with random seed
    
    Pass 128 bytes of random seed via FDT, so that guests' CRNGs are better 
seeded
    early at boot. This is larger than ChaCha20 key size of 32, so each byte of
    CRNG state will be mixed 4 times using this seed. There does not seem to be
    advantage in larger seed though.
    
    Depending on its configuration Linux can use the seed as device randomness
    or to just quickly initialize CRNG.
    In either case this will provide extra randomness to further harden CRNG.
    
    Signed-off-by: Sergiy Kibrik <Sergiy_Kibrik@xxxxxxxx>
    Reviewed-by: Julien Grall <julien@xxxxxxx>
    Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
---
 tools/libs/light/libxl_arm.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/tools/libs/light/libxl_arm.c b/tools/libs/light/libxl_arm.c
index e2901f13b7..e3140a6e00 100644
--- a/tools/libs/light/libxl_arm.c
+++ b/tools/libs/light/libxl_arm.c
@@ -305,6 +305,9 @@ static int make_chosen_node(libxl__gc *gc, void *fdt, bool 
ramdisk,
 {
     int res;
 
+    /* 1024 bit enough to mix Linux CRNG state several times */
+    uint8_t seed[128];
+
     /* See linux Documentation/devicetree/... */
     res = fdt_begin_node(fdt, "chosen");
     if (res) return res;
@@ -343,6 +346,11 @@ static int make_chosen_node(libxl__gc *gc, void *fdt, bool 
ramdisk,
         if (res) return res;
     }
 
+    res = libxl__random_bytes(gc, seed, sizeof(seed));
+    if (res) return res;
+    res = fdt_property(fdt, "rng-seed", seed, sizeof(seed));
+    if (res) return res;
+
     res = fdt_end_node(fdt);
     if (res) return res;
 
--
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