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um: fix execve stub execution on old host OSs

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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: 32e8eaf263 ("um: use execveat to create userspace MMs")
Reported-by: Glenn Washburn <development@efficientek.com>
Closes: https://lore.kernel.org/20250108022404.05e0de1e@crass-HP-ZBook-15-G2
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20250113094107.674738-1-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
This commit is contained in:
Benjamin Berg 2025-01-13 10:41:07 +01:00 • committed by Richard Weinberger
parent 3c2fc7434d
commit f82a9e7b9f
1 changed files with 13 additions and 3 deletions
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16
arch/um/os-Linux/skas/process.c
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@ -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);
}