Friendly ping. :) On Wed, Sep 01, 2021 at 12:42:08PM -0700, Kees Cook wrote: > I'm doing some thread necromancy of > https://lore.kernel.org/lkml/202007081624.82FA0CC1EA@keescook/ > > x86, arm64, and arm32 adjusted their READ_IMPLIES_EXEC logic to better > align with the safer defaults and the interactions with other mappings, > which I illustrated with this comment on x86: > > /* > * An executable for which elf_read_implies_exec() returns TRUE will > * have the READ_IMPLIES_EXEC personality flag set automatically. > * > * The decision process for determining the results are: > * > * CPU: | lacks NX* | has NX, ia32 | has NX, x86_64 | > * ELF: | | | | > * ---------------------|------------|------------------|----------------| > * missing PT_GNU_STACK | exec-all | exec-all | exec-none | > * PT_GNU_STACK == RWX | exec-stack | exec-stack | exec-stack | > * PT_GNU_STACK == RW | exec-none | exec-none | exec-none | > * > * exec-all : all PROT_READ user mappings are executable, except when > * backed by files on a noexec-filesystem. > * exec-none : only PROT_EXEC user mappings are executable. > * exec-stack: only the stack and PROT_EXEC user mappings are > * executable. > * > * *this column has no architectural effect: NX markings are ignored by > * hardware, but may have behavioral effects when "wants X" collides with > * "cannot be X" constraints in memory permission flags, as in > * https://lkml.kernel.org/r/20190418055759.GA3155@xxxxxxxxxxxx > * > */ > > For MIPS, the "lacks NX" above is the "!cpu_has_rixi" check. On x86, > we decided that the READ_IMPLIES_EXEC flag needed to reflect the > expectations, not the architectural behavior due to bad interactions > as noted above, as always returning "1" on non-NX hardware breaks > some mappings. > > The other part of the issue is "what does the MIPS toolchain do for > PT_GNU_STACK?" The answer seems to be "by default, include PT_GNU_STACK, > but mark it executable" (likely due to concerns over non-NX hardware): > > $ mipsel-linux-gnu-gcc -o hello_world hello_world.c > $ llvm-readelf -lW hellow_world | grep GNU_STACK > GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RWE 0x10 > > Given that older hardware doesn't support non-executable memory, it > seems safe to make the "PT_GNU_STACK is absent" logic mean "assume > non-executable", but this might break very old software running on > modern MIPS. This situation matches the ia32-on-x86_64 logic x86 > uses (which assumes needing READ_IMPLIES_EXEC in that situation). But > modern toolchains on modern MIPS hardware should follow a safer default > (assume NX stack). > > A follow-up to this change would be to switch the MIPS toolchain to emit > a non-executable PT_GNU_STACK, as this seems to be unneeded. > > Cc: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx> > Cc: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx> > Cc: Xuefeng Li <lixuefeng@xxxxxxxxxxx> > Cc: Juxin Gao <gaojuxin@xxxxxxxxxxx> > Cc: linux-mips@xxxxxxxxxxxxxxx > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > --- > arch/mips/kernel/elf.c | 16 +++++----------- > 1 file changed, 5 insertions(+), 11 deletions(-) > > diff --git a/arch/mips/kernel/elf.c b/arch/mips/kernel/elf.c > index 7b045d2a0b51..5582a4ca1e9e 100644 > --- a/arch/mips/kernel/elf.c > +++ b/arch/mips/kernel/elf.c > @@ -328,16 +328,10 @@ void mips_set_personality_nan(struct arch_elf_state *state) > > int mips_elf_read_implies_exec(void *elf_ex, int exstack) > { > - if (exstack != EXSTACK_DISABLE_X) { > - /* The binary doesn't request a non-executable stack */ > - return 1; > - } > - > - if (!cpu_has_rixi) { > - /* The CPU doesn't support non-executable memory */ > - return 1; > - } > - > - return 0; > + /* > + * Set READ_IMPLIES_EXEC only on non-NX systems that > + * do not request a specific state via PT_GNU_STACK. > + */ > + return (!cpu_has_rixi && exstack == EXSTACK_DEFAULT); > } > EXPORT_SYMBOL(mips_elf_read_implies_exec); > -- > 2.30.2 > -- Kees Cook
