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SubjectRe: [PATCH v2] powerpc/uprobes: Validation for prefixed instruction
On 2021/02/04 04:17PM, Ravi Bangoria wrote:
> Don't allow Uprobe on 2nd word of a prefixed instruction. As per
> ISA 3.1, prefixed instruction should not cross 64-byte boundary.
> So don't allow Uprobe on such prefixed instruction as well.
>
> There are two ways probed instruction is changed in mapped pages.
> First, when Uprobe is activated, it searches for all the relevant
> pages and replace instruction in them. In this case, if we notice
> that probe is on the 2nd word of prefixed instruction, error out
> directly. Second, when Uprobe is already active and user maps a
> relevant page via mmap(), instruction is replaced via mmap() code
> path. But because Uprobe is invalid, entire mmap() operation can
> not be stopped. In this case just print an error and continue.
>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
> ---
> v1: http://lore.kernel.org/r/20210119091234.76317-1-ravi.bangoria@linux.ibm.com
> v1->v2:
> - Instead of introducing new arch hook from verify_opcode(), use
> existing hook arch_uprobe_analyze_insn().
> - Add explicit check for prefixed instruction crossing 64-byte
> boundary. If probe is on such instruction, throw an error.
>
> arch/powerpc/kernel/uprobes.c | 66 ++++++++++++++++++++++++++++++++++-
> 1 file changed, 65 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kernel/uprobes.c b/arch/powerpc/kernel/uprobes.c
> index e8a63713e655..485d19a2a31f 100644
> --- a/arch/powerpc/kernel/uprobes.c
> +++ b/arch/powerpc/kernel/uprobes.c
> @@ -7,6 +7,7 @@
> * Adapted from the x86 port by Ananth N Mavinakayanahalli <ananth@in.ibm.com>
> */
> #include <linux/kernel.h>
> +#include <linux/highmem.h>
> #include <linux/sched.h>
> #include <linux/ptrace.h>
> #include <linux/uprobes.h>
> @@ -28,6 +29,69 @@ bool is_trap_insn(uprobe_opcode_t *insn)
> return (is_trap(*insn));
> }
>
> +#ifdef CONFIG_PPC64
> +static int get_instr(struct mm_struct *mm, unsigned long addr, u32 *instr)
> +{
> + struct page *page;
> + struct vm_area_struct *vma;
> + void *kaddr;
> + unsigned int gup_flags = FOLL_FORCE | FOLL_SPLIT_PMD;
> +
> + if (get_user_pages_remote(mm, addr, 1, gup_flags, &page, &vma, NULL) <= 0)
> + return -EINVAL;
> +
> + kaddr = kmap_atomic(page);
> + *instr = *((u32 *)(kaddr + (addr & ~PAGE_MASK)));
> + kunmap_atomic(kaddr);
> + put_page(page);
> + return 0;
> +}
> +
> +static int validate_prefixed_instr(struct mm_struct *mm, unsigned long addr)
> +{
> + struct ppc_inst inst;
> + u32 prefix, suffix;
> +
> + /*
> + * No need to check if addr is pointing to beginning of the
> + * page. Even if probe is on a suffix of page-unaligned
> + * prefixed instruction, hw will raise exception and kernel
> + * will send SIGBUS.
> + */
> + if (!(addr & ~PAGE_MASK))
> + return 0;
> +
> + if (get_instr(mm, addr, &prefix) < 0)
> + return -EINVAL;
> + if (get_instr(mm, addr + 4, &suffix) < 0)
> + return -EINVAL;
> +
> + inst = ppc_inst_prefix(prefix, suffix);
> + if (ppc_inst_prefixed(inst) && (addr & 0x3F) == 0x3C) {
> + printk_ratelimited("Cannot register a uprobe on 64 byte "
^^^^^^^^^^^^^^^^^^ pr_info_ratelimited()

It should be sufficient to check the primary opcode to determine if it
is a prefixed instruction. You don't have to read the suffix. I see that
we don't have a helper to do this currently, so you could do:

if (ppc_inst_primary_opcode(ppc_inst(prefix)) == 1)

In the future, it might be worthwhile to add IS_PREFIX() as a macro
similar to IS_MTMSRD() if there are more such uses.

Along with this, if you also add the below to the start of this
function, you can get rid of the #ifdef:

if (!IS_ENABLED(CONFIG_PPC64))
return 0;


- Naveen

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