In the Linux kernel, the following vulnerability has been resolved:
riscv: fix runtime constant support for nommu kernels
the __runtime_fixup_32 function does not handle the case where val is
zero correctly (as might occur when patching a nommu kernel and referring
to a physical address below the 4GiB boundary whose upper 32 bits are all
zero) because nothing in the existing logic prevents the code from taking
the else branch of both nop-checks and emitting two nop instructions.
This leaves random garbage in the register that is supposed to receive the
upper 32 bits of the pointer instead of zero that when combined with the
value for the lower 32 bits yields an invalid pointer and causes a kernel
panic when that pointer is eventually accessed.
The author clearly considered the fact that if the lui is converted into
a nop that the second instruction needs to be adjusted to become an li
instead of an addi, hence introducing the addi_insn_mask variable, but
didn't follow that logic through fully to the case where the else branch
executes. To fix it just adjust the logic to ensure that the second else
branch is not taken if the first instruction will be patched to a nop.
References
In the Linux kernel, the following vulnerability has been resolved:
riscv: fix runtime constant support for nommu kernels
the
__runtime_fixup_32function does not handle the case wherevaliszero correctly (as might occur when patching a nommu kernel and referring
to a physical address below the 4GiB boundary whose upper 32 bits are all
zero) because nothing in the existing logic prevents the code from taking
the
elsebranch of both nop-checks and emitting twonopinstructions.This leaves random garbage in the register that is supposed to receive the
upper 32 bits of the pointer instead of zero that when combined with the
value for the lower 32 bits yields an invalid pointer and causes a kernel
panic when that pointer is eventually accessed.
The author clearly considered the fact that if the
luiis converted intoa
nopthat the second instruction needs to be adjusted to become anliinstead of an
addi, hence introducing theaddi_insn_maskvariable, butdidn't follow that logic through fully to the case where the
elsebranchexecutes. To fix it just adjust the logic to ensure that the second
elsebranch is not taken if the first instruction will be patched to a
nop.References