Re: [PATCH v4 3/4] livepatch: refuse to resolve symbols that belong to init sections

[Roger Pau Monné]

On Wed, Apr 24, 2024 at 10:19:56AM +0200, Roger Pau Monne wrote:
> Livepatch payloads containing symbols that belong to init sections can only
> lead to page faults later on, as by the time the livepatch is loaded init
> sections have already been freed.
> 
> Refuse to resolve such symbols and return an error instead.
> 
> Note such resolutions are only relevant for symbols that point to undefined
> sections (SHN_UNDEF), as that implies the symbol is not in the current payload
> and hence must either be a Xen or a different livepatch payload symbol.
> 
> Do not allow to resolve symbols that point to __init_begin, as that address is
> also unmapped.  On the other hand, __init_end is not unmapped, and hence allow
> resolutions against it.
> 
> Since __init_begin can alias other symbols (like _erodata for example)
> allow the force flag to override the check and resolve the symbol anyway.

I've been thinking more about this, and it has further corner cases,
for example with this patch applied attempting to livepatch a function
that contains a rune like:

if ( system_state < SYS_STATE_active )
call_init_function();

Will now fail as Xen will refuse to resolve the call_init_function
symbol.

I however don't have any better ideas about how to solve this.  Maybe
it's fine to require such patches to use the --force option, as
refusing to resolve init symbols adds a bit more safety to
livepatches.

Thanks, Roger.

[PATCH v4 3/4] livepatch: refuse to resolve symbols that belong to init sections

[Roger Pau Monne]

Livepatch payloads containing symbols that belong to init sections can only
lead to page faults later on, as by the time the livepatch is loaded init
sections have already been freed.

Refuse to resolve such symbols and return an error instead.

Note such resolutions are only relevant for symbols that point to undefined
sections (SHN_UNDEF), as that implies the symbol is not in the current payload
and hence must either be a Xen or a different livepatch payload symbol.

Do not allow to resolve symbols that point to __init_begin, as that address is
also unmapped.  On the other hand, __init_end is not unmapped, and hence allow
resolutions against it.

Since __init_begin can alias other symbols (like _erodata for example)
allow the force flag to override the check and resolve the symbol anyway.

Signed-off-by: Roger Pau Monné 
---
Changes since v3:
 - Print warning message even when using the force option.

Changes since v2:
 - Allow bypassing added check with the force flag.

Changes since v1:
 - Fix off-by-one in range checking.
---
 xen/common/livepatch.c  | 13 -
 xen/common/livepatch_elf.c  | 22 +-
 xen/include/xen/livepatch_elf.h |  2 +-
 3 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/xen/common/livepatch.c b/xen/common/livepatch.c
index 502e264bc6fe..1afde0281402 100644
--- a/xen/common/livepatch.c
+++ b/xen/common/livepatch.c
@@ -1080,7 +1080,8 @@ static void free_payload(struct payload *data)
 xfree(data);
 }
 
-static int load_payload_data(struct payload *payload, void *raw, size_t len)
+static int load_payload_data(struct payload *payload, void *raw, size_t len,
+ bool force)
 {
 struct livepatch_elf elf = { .name = payload->name, .len = len };
 int rc = 0;
@@ -1093,7 +1094,7 @@ static int load_payload_data(struct payload *payload, 
void *raw, size_t len)
 if ( rc )
 goto out;
 
-rc = livepatch_elf_resolve_symbols();
+rc = livepatch_elf_resolve_symbols(, force);
 if ( rc )
 goto out;
 
@@ -1133,7 +1134,8 @@ static int load_payload_data(struct payload *payload, 
void *raw, size_t len)
 return rc;
 }
 
-static int livepatch_upload(struct xen_sysctl_livepatch_upload *upload)
+static int livepatch_upload(struct xen_sysctl_livepatch_upload *upload,
+bool force)
 {
 struct payload *data, *found;
 char n[XEN_LIVEPATCH_NAME_SIZE];
@@ -1162,7 +1164,7 @@ static int livepatch_upload(struct 
xen_sysctl_livepatch_upload *upload)
 {
 memcpy(data->name, n, strlen(n));
 
-rc = load_payload_data(data, raw_data, upload->size);
+rc = load_payload_data(data, raw_data, upload->size, force);
 if ( rc )
 goto out;
 
@@ -2132,7 +2134,8 @@ int livepatch_op(struct xen_sysctl_livepatch_op 
*livepatch)
 switch ( livepatch->cmd )
 {
 case XEN_SYSCTL_LIVEPATCH_UPLOAD:
-rc = livepatch_upload(>u.upload);
+rc = livepatch_upload(>u.upload,
+  livepatch->flags & LIVEPATCH_FLAG_FORCE);
 break;
 
 case XEN_SYSCTL_LIVEPATCH_GET:
diff --git a/xen/common/livepatch_elf.c b/xen/common/livepatch_elf.c
index 45d73912a3cd..1d0ed8f99bcc 100644
--- a/xen/common/livepatch_elf.c
+++ b/xen/common/livepatch_elf.c
@@ -4,6 +4,7 @@
 
 #include 
 #include 
+#include 
 #include 
 #include 
 #include 
@@ -276,7 +277,7 @@ static int elf_get_sym(struct livepatch_elf *elf, const 
void *data)
 return 0;
 }
 
-int livepatch_elf_resolve_symbols(struct livepatch_elf *elf)
+int livepatch_elf_resolve_symbols(struct livepatch_elf *elf, bool force)
 {
 unsigned int i;
 int rc = 0;
@@ -310,6 +311,25 @@ int livepatch_elf_resolve_symbols(struct livepatch_elf 
*elf)
 break;
 }
 }
+
+/*
+ * Ensure not an init symbol.  Only applicable to Xen symbols, as
+ * livepatch payloads don't have init sections or equivalent.
+ */
+else if ( st_value >= (uintptr_t)&__init_begin &&
+  st_value <  (uintptr_t)&__init_end )
+{
+printk("%s" LIVEPATCH "%s: symbol %s is in init section%s\n",
+   force ? XENLOG_WARNING : XENLOG_ERR,
+   elf->name, elf->sym[i].name,
+   force ? "" : ", not resolving");
+if ( !force )
+{
+rc = -ENXIO;
+break;
+}
+}
+
 dprintk(XENLOG_DEBUG, LIVEPATCH "%s: Undefined symbol resolved: %s 
=> %#"PRIxElfAddr"\n",
 elf->name, elf->sym[i].name, st_value);
 break;
diff --git a/xen/include/xen/livepatch_elf.h b/xen/include/xen/livepatch_elf.h
index 7116deaddc28..84e9c5eb7be5 100644
--- a/xen/include/xen/livepatch_elf.h
+++ b/xen/include/xen/livepatch_elf.h
@@ -44,7 +44,7 @@ livepatch_elf_sec_by_name(const struct