executor: disable /dev/[k]?mem access through CAP_SYS_RAWIO #6322
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Currently, syz-kconf disables CONFIG_DEVMEM and CONFIG_DEVKMEM but on some setups, these nodes might be needed by various system daemons. To allow these daemons to work while fuzzing, we need to re-enable those CONFIGs. On the other hand, we really don't want fuzzing to break the machine by accessing these nodes. Since their access is guarded by a capability, we can have syz-executor drop that capability as part of the shared "drop_caps()" logic. That capability has a slightly larger scope than guarding /dev/mem and /dev/kmem but it seems to me that the rest is all equally risky and could break the system in all sorts of unexpected way so dropping the capability seems to be the right thing to do anyway.
a-nogikh
reviewed
Aug 25, 2025
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Currently, syz-kconf disables CONFIG_DEVMEM and CONFIG_DEVKMEM but on some setups, these nodes might be needed by various system daemons. To allow these daemons to work while fuzzing, we need to re-enable those CONFIGs.
On the other hand, we really don't want fuzzing to break the machine by accessing these nodes. Since their access is guarded by a capability, we can have syz-executor drop that capability as part of the shared "drop_caps()" logic. That capability has a slightly larger scope than /dev/mem and /dev/kmem but it seems to me that these are all equally risky low level operations which could break the system in all sorts of unexpected way and dropping the capability seems safer.