On Mon, 13 Jan 2025 10:41:07 +0100
Benjamin Berg <benja...@sipsolutions.net> wrote:
> From: Benjamin Berg <benjamin.b...@intel.com>
>
> The stub execution uses the somewhat new close_range and execveat
> syscalls. Of these two, the execveat call is essential, but the
> close_range call is more about stub process hygiene rather than safety
> (and its result is ignored).
>
> Replace both calls with a raw syscall as older machines might not have a
> recent enough kernel for close_range (with CLOSE_RANGE_CLOEXEC) or a
> libc that does not yet expose both of the syscalls.
>
> Fixes: 32e8eaf263d9 ("um: use execveat to create userspace MMs")
> Reported-by: Glenn Washburn <developm...@efficientek.com>
> Closes: https://lore.kernel.org/20250108022404.05e0de1e@crass-HP-ZBook-15-G2
> Signed-off-by: Benjamin Berg <benjamin.b...@intel.com>
Tested-by: Glenn Washburn <developm...@efficientek.com>
>
> ---
>
> v2:
> - Fix regression in previous version to not close FD 0
> ---
> arch/um/os-Linux/skas/process.c | 16 +++++++++++++---
> 1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
> index f683cfc9e51a..e2f8f156402f 100644
> --- a/arch/um/os-Linux/skas/process.c
> +++ b/arch/um/os-Linux/skas/process.c
> @@ -181,6 +181,10 @@ extern char __syscall_stub_start[];
>
> static int stub_exe_fd;
>
> +#ifndef CLOSE_RANGE_CLOEXEC
> +#define CLOSE_RANGE_CLOEXEC (1U << 2)
> +#endif
> +
> static int userspace_tramp(void *stack)
> {
> char *const argv[] = { "uml-userspace", NULL };
> @@ -202,8 +206,12 @@ static int userspace_tramp(void *stack)
> init_data.stub_data_fd = phys_mapping(uml_to_phys(stack), &offset);
> init_data.stub_data_offset = MMAP_OFFSET(offset);
>
> - /* Set CLOEXEC on all FDs and then unset on all memory related FDs */
> - close_range(0, ~0U, CLOSE_RANGE_CLOEXEC);
> + /*
> + * Avoid leaking unneeded FDs to the stub by setting CLOEXEC on all FDs
> + * and then unsetting it on all memory related FDs.
> + * This is not strictly necessary from a safety perspective.
> + */
> + syscall(__NR_close_range, 0, ~0U, CLOSE_RANGE_CLOEXEC);
>
> fcntl(init_data.stub_data_fd, F_SETFD, 0);
> for (iomem = iomem_regions; iomem; iomem = iomem->next)
> @@ -224,7 +232,9 @@ static int userspace_tramp(void *stack)
> if (ret != sizeof(init_data))
> exit(4);
>
> - execveat(stub_exe_fd, "", argv, NULL, AT_EMPTY_PATH);
> + /* Raw execveat for compatibility with older libc versions */
> + syscall(__NR_execveat, stub_exe_fd, (unsigned long)"",
> + (unsigned long)argv, NULL, AT_EMPTY_PATH);
>
> exit(5);
> }
