x86/um: Do not inherit vDSO from host

Inheriting the vDSO from the host is problematic. The values read
from the time functions will not be correct for the UML kernel.
Furthermore the start and end of the vDSO are not stable or
detectable by userspace. Specifically the vDSO datapages start
before AT_SYSINFO_EHDR and the vDSO itself is larger than a single page.

This codepath is only used on 32bit x86 UML. In my testing with both
32bit and 64bit hosts the passthrough functionality has always been
disabled anyways due to the checks against envp in scan_elf_aux().

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://patch.msgid.link/20251028-uml-remove-32bit-pseudo-vdso-v1-4-e930063eff5f@weissschuh.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: t-8ch

arch/um/os-Linux/elf_aux.c

@@ -20,31 +20,15 @@ typedef Elf32_auxv_t elf_auxv_t;
 /* These are initialized very early in boot and never changed */
 char * elf_aux_platform;
 extern long elf_aux_hwcap;
-unsigned long vsyscall_ehdr;
-unsigned long vsyscall_end;
-unsigned long __kernel_vsyscall;
 
 __init void scan_elf_aux( char **envp)
 {
-	long page_size = 0;
 	elf_auxv_t * auxv;
 
 	while ( *envp++ != NULL) ;
 
 	for ( auxv = (elf_auxv_t *)envp; auxv->a_type != AT_NULL; auxv++) {
 		switch ( auxv->a_type ) {
-			case AT_SYSINFO:
-				__kernel_vsyscall = auxv->a_un.a_val;
-				/* See if the page is under TASK_SIZE */
-				if (__kernel_vsyscall < (unsigned long) envp)
-					__kernel_vsyscall = 0;
-				break;
-			case AT_SYSINFO_EHDR:
-				vsyscall_ehdr = auxv->a_un.a_val;
-				/* See if the page is under TASK_SIZE */
-				if (vsyscall_ehdr < (unsigned long) envp)
-					vsyscall_ehdr = 0;
-				break;
 			case AT_HWCAP:
 				elf_aux_hwcap = auxv->a_un.a_val;
 				break;
@@ -56,17 +40,6 @@ __init void scan_elf_aux( char **envp)
 				elf_aux_platform =
 					(char *) (long) auxv->a_un.a_val;
 				break;
-			case AT_PAGESZ:
-				page_size = auxv->a_un.a_val;
-				break;
 		}
 	}
-	if ( ! __kernel_vsyscall || ! vsyscall_ehdr ||
-	     ! page_size || (vsyscall_ehdr % page_size) ) {
-		__kernel_vsyscall = 0;
-		vsyscall_ehdr = 0;
-	}
-	else {
-		vsyscall_end = vsyscall_ehdr + page_size;
-	}
 }

arch/um/os-Linux/user_syms.c

@@ -31,12 +31,6 @@ extern void *memset(void *, int, size_t);
 EXPORT_SYMBOL(memset);
 #endif
 
-#ifdef CONFIG_ARCH_REUSE_HOST_VSYSCALL_AREA
-/* needed for __access_ok() */
-EXPORT_SYMBOL(vsyscall_ehdr);
-EXPORT_SYMBOL(vsyscall_end);
-#endif
-
 #ifdef _FORTIFY_SOURCE
 extern int __sprintf_chk(char *str, int flag, size_t len, const char *format);
 EXPORT_SYMBOL(__sprintf_chk);

arch/x86/um/Kconfig

@@ -8,7 +8,6 @@ endmenu
 
 config UML_X86
 	def_bool y
-	select ARCH_BINFMT_ELF_EXTRA_PHDRS if X86_32
 	select ARCH_USE_QUEUED_RWLOCKS
 	select ARCH_USE_QUEUED_SPINLOCKS
 	select DCACHE_WORD_ACCESS

arch/x86/um/Makefile

@@ -18,7 +18,6 @@ obj-y = bugs_$(BITS).o delay.o fault.o \
 ifeq ($(CONFIG_X86_32),y)
 
 obj-y += syscalls_32.o
-obj-$(CONFIG_ELF_CORE) += elfcore.o
 
 subarch-y = ../lib/string_32.o ../lib/atomic64_32.o ../lib/atomic64_cx8_32.o
 subarch-y += ../lib/cmpxchg8b_emu.o ../lib/atomic64_386_32.o

arch/x86/um/asm/elf.h

@@ -72,32 +72,9 @@ extern char * elf_aux_platform;
 #define ELF_PLATFORM_FALLBACK "i586"
 #define ELF_PLATFORM (elf_aux_platform ?: ELF_PLATFORM_FALLBACK)
 
-extern unsigned long vsyscall_ehdr;
-extern unsigned long vsyscall_end;
-extern unsigned long __kernel_vsyscall;
-
-/*
- * This is the range that is readable by user mode, and things
- * acting like user mode such as get_user_pages.
- */
-#define FIXADDR_USER_START      vsyscall_ehdr
-#define FIXADDR_USER_END        vsyscall_end
-
-
-/*
- * Architecture-neutral AT_ values in 0-17, leave some room
- * for more of them, start the x86-specific ones at 32.
- */
-#define AT_SYSINFO		32
-#define AT_SYSINFO_EHDR		33
-
-#define ARCH_DLINFO						\
-do {								\
-	if ( vsyscall_ehdr ) {					\
-		NEW_AUX_ENT(AT_SYSINFO,	__kernel_vsyscall);	\
-		NEW_AUX_ENT(AT_SYSINFO_EHDR, vsyscall_ehdr);	\
-	}							\
-} while (0)
+/* No user-accessible fixmap addresses, i.e. vsyscall */
+#define FIXADDR_USER_START      0
+#define FIXADDR_USER_END        0
 
 #else