[PATCH v2] um: fix execve stub execution on old host OSs

[Benjamin Berg <benjamin@sipsolutions.net>]

From: Benjamin Berg <benjamin.berg@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 <development@efficientek.com>
Closes: https://lore.kernel.org/20250108022404.05e0de1e@crass-HP-ZBook-15-G2
Signed-off-by: Benjamin Berg <benjamin.berg@intel.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);
 }
-- 
2.47.1