Messages in this thread | | | Date | Fri, 17 Oct 2014 12:40:00 -0500 | Subject | Re: [PATCH v6 8/8] ARM: mm: allow text and rodata sections to be read-only | From | Kees Cook <> |
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On Fri, Oct 17, 2014 at 1:15 AM, Hillf Danton <dhillf@gmail.com> wrote: > Hey Kees > >> From: Kees Cook <keescook@chromium.org> >> To: linux-kernel@vger.kernel.org >> Cc: Kees Cook <keescook@chromium.org>, Will Deacon <will.deacon@arm.com>, >> Rabin Vincent <rabin@rab.in>, Laura Abbott <lauraa@codeaurora.org>, Rob >> Herring <robh@kernel.org>, Leif Lindholm <leif.lindholm@linaro.org>, Mark >> Salter <msalter@redhat.com>, Liu hua < >> Subject: [PATCH v6 8/8] ARM: mm: allow text and rodata sections to be >> read-only >> Date: Thu, 18 Sep 2014 12:19:09 -0700 >> >> This introduces CONFIG_DEBUG_RODATA, making kernel text and rodata >> read-only. Additionally, this splits rodata from text so that rodata can >> also be NX, which may lead to wasted memory when aligning to SECTION_SIZE. >> The read-only areas are made writable during ftrace updates and kexec. >> >> Signed-off-by: Kees Cook <keescook@chromium.org> >> Tested-by: Laura Abbott <lauraa@codeaurora.org> >> Acked-by: Nicolas Pitre <nico@linaro.org> >> --- >> [...] >> +#ifdef CONFIG_DEBUG_RODATA >> +static struct section_perm ro_perms[] = { >> + /* Make kernel code and rodata RX (set RO). */ >> + { >> + .start = (unsigned long)_stext, >> + .end = (unsigned long)__init_begin, >> +#ifdef CONFIG_ARM_LPAE >> + .mask = ~PMD_SECT_RDONLY, >> + .prot = PMD_SECT_RDONLY, >> +#else >> + .mask = ~(PMD_SECT_APX | PMD_SECT_AP_WRITE), >> + .prot = PMD_SECT_APX | PMD_SECT_AP_WRITE, >> + .clear = PMD_SECT_AP_WRITE, > > [1] after set_kernel_text_ro, PMD_SECT_AP_WRITE is set > in corresponding PMD. > > [2] after set_kernel_text_rw, PMD_SECT_AP_WRITE is also set > in corresponding PMD. > > as shown in [7/8] in this patch series. > > +#ifdef CONFIG_ARM_LPAE > + pmd[0] = __pmd((pmd_val(pmd[0]) & mask) | prot); > +#else > + if (addr & SECTION_SIZE) > + pmd[1] = __pmd((pmd_val(pmd[1]) & mask) | prot); > + else > + pmd[0] = __pmd((pmd_val(pmd[0]) & mask) | prot); > +#endif > > [3] As far as I could understand, > I wonder if PMD_SECT_AP_WRITE should be cleared > when setting RO.
The bit names are confusing. PMD_SECT_APX is what actually controls the ROness. As in, the region is read-only when PMD_SECT_APX=1 and PMD_SECT_AP_WRITE=1, and writable when PMD_SECT_APX=0 and PMD_SECT_AP_WRITE=1.
> btw, would you please shed light on PMD_SECT_APX, > since it is not included in both MT_MEMORY_RWX and > MT_MEMORY_RW mm types. > > [MT_MEMORY_RWX] = { > .prot_pte = L_PTE_PRESENT | L_PTE_YOUNG | L_PTE_DIRTY, > .prot_l1 = PMD_TYPE_TABLE, > .prot_sect = PMD_TYPE_SECT | PMD_SECT_AP_WRITE, > .domain = DOMAIN_KERNEL, > }, > [MT_MEMORY_RW] = { > .prot_pte = L_PTE_PRESENT | L_PTE_YOUNG | L_PTE_DIRTY | > L_PTE_XN, > .prot_l1 = PMD_TYPE_TABLE, > .prot_sect = PMD_TYPE_SECT | PMD_SECT_AP_WRITE, > .domain = DOMAIN_KERNEL, > },
Correct, both of those are read-write, so PMD_SECT_APX is not set. If there were an MT_MEMORY_RO type, it would have PMD_SECT_APX set. For example, see that MT_ROM gets it set when extended page tables are available (which the ro-nx code similarly checks via arch_has_strict_perms()).
I hope that helps!
-Kees
-- Kees Cook Chrome OS Security
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