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x86: Add magic AMD return-thunk

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Note: needs to be in a section distinct from Retpolines such that the
Retpoline RET substitution cannot possibly use immediate jumps.

ORC unwinding for zen_untrain_ret() and __x86_return_thunk() is a
little tricky but works due to the fact that zen_untrain_ret() doesn't
have any stack ops and as such will emit a single ORC entry at the
start (+0x3f).

Meanwhile, unwinding an IP, including the __x86_return_thunk() one
(+0x40) will search for the largest ORC entry smaller or equal to the
IP, these will find the one ORC entry (+0x3f) and all works.

  [ Alexandre: SVM part. ]
  [ bp: Build fix, massages. ]

Suggested-by: Andrew Cooper <Andrew.Cooper3@citrix.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
This commit is contained in:
Peter Zijlstra 2022-06-14 23:15:48 +02:00 • committed by Borislav Petkov
parent 951ddecf43
commit a149180fbc
9 changed files with 128 additions and 9 deletions
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22
tools/objtool/check.c
View file

@ -1302,7 +1302,7 @@ static void add_retpoline_call(struct objtool_file *file, struct instruction *in
annotate_call_site(file, insn, false);
}
static void add_return_call(struct objtool_file *file, struct instruction *insn)
static void add_return_call(struct objtool_file *file, struct instruction *insn, bool add)
{
/*
* Return thunk tail calls are really just returns in disguise,
@ -1311,7 +1311,8 @@ static void add_return_call(struct objtool_file *file, struct instruction *insn)
insn->type = INSN_RETURN;
insn->retpoline_safe = true;
list_add_tail(&insn->call_node, &file->return_thunk_list);
if (add)
list_add_tail(&insn->call_node, &file->return_thunk_list);
}
static bool same_function(struct instruction *insn1, struct instruction *insn2)
@ -1367,7 +1368,7 @@ static int add_jump_destinations(struct objtool_file *file)
add_retpoline_call(file, insn);
continue;
} else if (reloc->sym->return_thunk) {
add_return_call(file, insn);
add_return_call(file, insn, true);
continue;
} else if (insn->func) {
/*
@ -1387,6 +1388,21 @@ static int add_jump_destinations(struct objtool_file *file)
jump_dest = find_insn(file, dest_sec, dest_off);
if (!jump_dest) {
struct symbol *sym = find_symbol_by_offset(dest_sec, dest_off);
/*
* This is a special case for zen_untrain_ret().
* It jumps to __x86_return_thunk(), but objtool
* can't find the thunk's starting RET
* instruction, because the RET is also in the
* middle of another instruction. Objtool only
* knows about the outer instruction.
*/
if (sym && sym->return_thunk) {
add_return_call(file, insn, false);
continue;
}
WARN_FUNC("can't find jump dest instruction at %s+0x%lx",
insn->sec, insn->offset, dest_sec->name,
dest_off);