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SubjectRe: [PATCH v13 10/20] kernel, arm64: untag user pointers in prctl_set_mm*
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On 20/03/2019 14:51, Andrey Konovalov wrote:
> This patch is a part of a series that extends arm64 kernel ABI to allow to
> pass tagged user pointers (with the top byte set to something else other
> than 0x00) as syscall arguments.
>
> prctl_set_mm() and prctl_set_mm_map() use provided user pointers for vma
> lookups and do some pointer comparisons to perform validation, which can
> only by done with untagged pointers.
>
> Untag user pointers in these functions for vma lookup and validity checks.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
> kernel/sys.c | 44 ++++++++++++++++++++++++++++++--------------
> 1 file changed, 30 insertions(+), 14 deletions(-)
>
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 12df0e5434b8..fe26ccf3c9e6 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1885,11 +1885,12 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
> * WARNING: we don't require any capability here so be very careful
> * in what is allowed for modification from userspace.
> */
> -static int validate_prctl_map(struct prctl_mm_map *prctl_map)
> +static int validate_prctl_map(struct prctl_mm_map *tagged_prctl_map)
> {
> unsigned long mmap_max_addr = TASK_SIZE;
> struct mm_struct *mm = current->mm;
> int error = -EINVAL, i;
> + struct prctl_mm_map prctl_map;
>
> static const unsigned char offsets[] = {
> offsetof(struct prctl_mm_map, start_code),
> @@ -1905,12 +1906,25 @@ static int validate_prctl_map(struct prctl_mm_map *prctl_map)
> offsetof(struct prctl_mm_map, env_end),
> };
>
> + memcpy(&prctl_map, tagged_prctl_map, sizeof(prctl_map));
> + prctl_map.start_code = untagged_addr(prctl_map.start_code);
> + prctl_map.end_code = untagged_addr(prctl_map.end_code);
> + prctl_map.start_data = untagged_addr(prctl_map.start_data);
> + prctl_map.end_data = untagged_addr(prctl_map.end_data);
> + prctl_map.start_brk = untagged_addr(prctl_map.start_brk);
> + prctl_map.brk = untagged_addr(prctl_map.brk);
> + prctl_map.start_stack = untagged_addr(prctl_map.start_stack);
> + prctl_map.arg_start = untagged_addr(prctl_map.arg_start);
> + prctl_map.arg_end = untagged_addr(prctl_map.arg_end);
> + prctl_map.env_start = untagged_addr(prctl_map.env_start);
> + prctl_map.env_end = untagged_addr(prctl_map.env_end);
> +
> /*
> * Make sure the members are not somewhere outside
> * of allowed address space.
> */
> for (i = 0; i < ARRAY_SIZE(offsets); i++) {
> - u64 val = *(u64 *)((char *)prctl_map + offsets[i]);
> + u64 val = *(u64 *)((char *)&prctl_map + offsets[i]);
>
> if ((unsigned long)val >= mmap_max_addr ||
> (unsigned long)val < mmap_min_addr)
> @@ -1921,8 +1935,8 @@ static int validate_prctl_map(struct prctl_mm_map *prctl_map)
> * Make sure the pairs are ordered.
> */
> #define __prctl_check_order(__m1, __op, __m2) \
> - ((unsigned long)prctl_map->__m1 __op \
> - (unsigned long)prctl_map->__m2) ? 0 : -EINVAL
> + ((unsigned long)prctl_map.__m1 __op \
> + (unsigned long)prctl_map.__m2) ? 0 : -EINVAL
> error = __prctl_check_order(start_code, <, end_code);
> error |= __prctl_check_order(start_data, <, end_data);
> error |= __prctl_check_order(start_brk, <=, brk);
> @@ -1937,23 +1951,24 @@ static int validate_prctl_map(struct prctl_mm_map *prctl_map)
> /*
> * @brk should be after @end_data in traditional maps.
> */
> - if (prctl_map->start_brk <= prctl_map->end_data ||
> - prctl_map->brk <= prctl_map->end_data)
> + if (prctl_map.start_brk <= prctl_map.end_data ||
> + prctl_map.brk <= prctl_map.end_data)
> goto out;
>
> /*
> * Neither we should allow to override limits if they set.
> */
> - if (check_data_rlimit(rlimit(RLIMIT_DATA), prctl_map->brk,
> - prctl_map->start_brk, prctl_map->end_data,
> - prctl_map->start_data))
> + if (check_data_rlimit(rlimit(RLIMIT_DATA), prctl_map.brk,
> + prctl_map.start_brk, prctl_map.end_data,
> + prctl_map.start_data))
> goto out;
>
> /*
> * Someone is trying to cheat the auxv vector.
> */
> - if (prctl_map->auxv_size) {
> - if (!prctl_map->auxv || prctl_map->auxv_size > sizeof(mm->saved_auxv))
> + if (prctl_map.auxv_size) {
> + if (!prctl_map.auxv || prctl_map.auxv_size >
> + sizeof(mm->saved_auxv))
> goto out;
> }
>
> @@ -1962,7 +1977,7 @@ static int validate_prctl_map(struct prctl_mm_map *prctl_map)
> * change /proc/pid/exe link: only local sys admin should
> * be allowed to.
> */
> - if (prctl_map->exe_fd != (u32)-1) {
> + if (prctl_map.exe_fd != (u32)-1) {
> if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN))
> goto out;
> }
> @@ -2120,13 +2135,14 @@ static int prctl_set_mm(int opt, unsigned long addr,
> if (opt == PR_SET_MM_AUXV)
> return prctl_set_auxv(mm, addr, arg4);
>
> - if (addr >= TASK_SIZE || addr < mmap_min_addr)
> + if (untagged_addr(addr) >= TASK_SIZE ||
> + untagged_addr(addr) < mmap_min_addr)
> return -EINVAL;
>
> error = -EINVAL;
>
> down_write(&mm->mmap_sem);
> - vma = find_vma(mm, addr);
> + vma = find_vma(mm, untagged_addr(addr));
>
> prctl_map.start_code = mm->start_code;
> prctl_map.end_code = mm->end_code;

I think this new version is consistent w.r.t. tagged/untagged pointer usage. However,
I also note that a significant change has been introduced: it is now possible to set
MM fields to tagged addresses (tags are ignored by validate_prctl_map()). I am not
opposed to this as such, but have you considered the implications? Does it make sense
to have a tagged value for e.g. prctl_map.arg_start? Is the kernel able to handle
tagged values in those fields? I have the feeling that it's safer to discard tags for
now, and if necessary allow them to be preserved later on.

Kevin

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