Re: [PATCH arm64-next v3] net: bpf: arm64: address randomize and write protect JIT code

[Daniel Borkmann]

On 09/15/2014 11:30 PM, Will Deacon wrote:
> Hi Daniel,
>
> One small comment than I forgot to mention before (see below).
>
> On Mon, Sep 15, 2014 at 09:20:23PM +0100, Daniel Borkmann wrote:
> > This is the ARM64 variant for 314beb9bcab ("x86: bpf_jit_comp: secure bpf
> > jit against spraying attacks").
> >
> > Thanks to commit 11d91a770f1f ("arm64: Add CONFIG_DEBUG_SET_MODULE_RONX
> > support") which added necessary infrastructure, we can now implement
> > RO marking of eBPF generated JIT image pages and randomize start offset
> > for the JIT code, so that it does not reside directly on a page boundary
> > anymore. Likewise, the holes are filled with illegal instructions: here
> > we use BRK #0x100 (opcode 0xd4202000) to trigger a fault in the kernel
> > (unallocated BRKs would trigger a fault through do_debug_exception). This
> > seems more reliable as we don't have a guaranteed undefined instruction
> > space on ARM64.
> >
> > This is basically the ARM64 variant of what we already have in ARM via
> > commit 55309dd3d4cd ("net: bpf: arm: address randomize and write protect
> > JIT code"). Moreover, this commit also presents a merge resolution due to
> > conflicts with commit 60a3b2253c41 ("net: bpf: make eBPF interpreter images
> > read-only") as we don't use kfree() in bpf_jit_free() anymore to release
> > the locked bpf_prog structure, but instead bpf_prog_unlock_free() through
> > a different allocator.
> >
> > JIT tested on aarch64 with BPF test suite.
> >
> > Reference: http://mainisusuallyafunction.blogspot.com/2012/11/attacking-hardened-linux-systems-with.html
> > Signed-off-by: Daniel Borkmann <dborkman@xxxxxxxxxx>
> > Cc: Zi Shen Lim <zlim.lnx@xxxxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> > Cc: Alexei Starovoitov <ast@xxxxxxxxxxxx>
> > ---
> >   v2->v3:
> >    - Use cpu_to_le32() as suggested by Zi/Will
> >   v1->v2:
> >    - Use brk insn as suggested by Catalin
> >   Note:
> >    - This patch depends on net-next being merged to mainline due
> >      to the mentioned merge conflict.
> >
> >   arch/arm64/net/bpf_jit_comp.c | 38 +++++++++++++++++++++++++++++---------
> >   1 file changed, 29 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> > index 7ae3354..4b71779 100644
> > --- a/arch/arm64/net/bpf_jit_comp.c
> > +++ b/arch/arm64/net/bpf_jit_comp.c
> > @@ -19,7 +19,6 @@
> >   #define pr_fmt(fmt) "bpf_jit: " fmt
> >
> >   #include <linux/filter.h>
> > -#include <linux/moduleloader.h>
> >   #include <linux/printk.h>
> >   #include <linux/skbuff.h>
> >   #include <linux/slab.h>
> > @@ -119,6 +118,15 @@ static inline int bpf2a64_offset(int bpf_to, int bpf_from,
> >   	return to - from;
> >   }
> >
> > +static void jit_fill_hole(void *area, unsigned int size)
> > +{
> > +	/* We use brk #0x100 to trigger a fault. */
> > +	u32 *ptr;
> > +	/* We are guaranteed to have aligned memory. */
> > +	for (ptr = area; size >= sizeof(u32); size -= sizeof(u32))
> > +		*ptr++ = cpu_to_le32(0xd4202000);
> > +}
>
> Please can you use the existing macros in debug-monitors.h for constructing
> this value? We have AARCH64_BREAK_MON to get the BRK encoding, then you can
> add an immediate to that file to reserve something for bpf. That way, we
> make sure that other parts of the kernel don't accidentally repurpose your
> fault code.

Okay, no problem, I'll do that and respin tomorrow.

Thanks,

Daniel
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