Commit f82a9e7b authored by Benjamin Berg's avatar Benjamin Berg Committed by Richard Weinberger
Browse files

um: fix execve stub execution on old host OSs 



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: 32e8eaf2 ("um: use execveat to create userspace MMs")
Reported-by: default avatarGlenn Washburn <development@efficientek.com>
Closes: https://lore.kernel.org/20250108022404.05e0de1e@crass-HP-ZBook-15-G2


Signed-off-by: default avatarBenjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20250113094107.674738-1-benjamin@sipsolutions.net


Signed-off-by: default avatarJohannes Berg <johannes.berg@intel.com>
Signed-off-by: default avatarRichard Weinberger <richard@nod.at>
parent 3c2fc743

arch/um/os-Linux/skas/process.c

+13 −3
Original line number Diff line number Diff line
@@ -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);

}