From f82a9e7b9fa922bb9cccb00aae684a27b79e6df7 Mon Sep 17 00:00:00 2001 From: Benjamin Berg Date: Mon, 13 Jan 2025 10:41:07 +0100 Subject: [PATCH] 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: 32e8eaf263d9 ("um: use execveat to create userspace MMs") Reported-by: Glenn Washburn Closes: https://lore.kernel.org/20250108022404.05e0de1e@crass-HP-ZBook-15-G2 Signed-off-by: Benjamin Berg Link: https://patch.msgid.link/20250113094107.674738-1-benjamin@sipsolutions.net Signed-off-by: Johannes Berg Signed-off-by: Richard Weinberger --- 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); }