[PATCH] x86/vdso: Error out if the vDSO isn't a valid DSO

[Andy Lutomirski <luto@kernel.org>]

Some distros has been playing with toolchain changes that can affect
the type of ELF objects built.  Occasionally, this goes wrong and
the vDSO ends up not being a DSO at all.  This causes the kernel to
end up broken in a surprisingly subtle way -- glibc apparently
silently ignores a vDSO that isn't a DSO, so everything works,
albeit slowly, until users try a different libc implementation.

Make the kernel build process a bit more robust: fail outright if
the vDSO isn't ET_DYN or is missing its PT_DYNAMIC segment.  I've
never seen this in an unmodified kernel.

See https://github.com/docker/docker/issues/23378

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/entry/vdso/vdso2c.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/entry/vdso/vdso2c.h b/arch/x86/entry/vdso/vdso2c.h
index 63a03bb91497..4f741192846d 100644
--- a/arch/x86/entry/vdso/vdso2c.h
+++ b/arch/x86/entry/vdso/vdso2c.h
@@ -22,6 +22,9 @@ static void BITSFUNC(go)(void *raw_addr, size_t raw_len,
 
 	ELF(Phdr) *pt = (ELF(Phdr) *)(raw_addr + GET_LE(&hdr->e_phoff));
 
+	if (hdr->e_type != ET_DYN)
+		fail("input is not a shared object\n");
+
 	/* Walk the segment table. */
 	for (i = 0; i < GET_LE(&hdr->e_phnum); i++) {
 		if (GET_LE(&pt[i].p_type) == PT_LOAD) {
@@ -49,6 +52,9 @@ static void BITSFUNC(go)(void *raw_addr, size_t raw_len,
 	if (stripped_len < load_size)
 		fail("stripped input is too short\n");
 
+	if (!dyn)
+		fail("input has no PT_DYNAMIC section -- your toolchain is buggy\n");
+
 	/* Walk the dynamic table */
 	for (i = 0; dyn + i < dyn_end &&
 		     GET_LE(&dyn[i].d_tag) != DT_NULL; i++) {
-- 
2.7.4