[PATCH v3] perf dso: Fix kallsyms DSO detection with fallback logic

[PATCH v3] perf dso: Fix kallsyms DSO detection with fallback logic

[Tanushree Shah]

The current kallsyms detection in dso__is_kallsyms() uses the
dso_binary_type enum which fixes the issue of kallsyms being cached in
the build-id cache for out-of-tree modules.

However, during build-id injection in perf record/inject, dso_binary_type
has not been explicitly set yet,so dso__binary_type() returns
DSO_BINARY_TYPE__NOT_FOUND instead of DSO_BINARY_TYPE__KALLSYMS for the
kernel DSO. The current check then fails to identify it as kallsyms,
causing build-id symlinks to not be created in ~/.debug/.build-id/ and
perf archive to fail with "Cannot stat" errors.

Steps to reproduce the issue:
1. rm -rf ~/.debug/.build-id
2. perf record sleep 1
3. perf archive

Fix by falling back to matching long_name against the known kallsyms
strings explicitly when binary_type is not yet set
(== DSO_BINARY_TYPE__NOT_FOUND). Use strcmp() for exact matching of
fixed names and strict validation for guest kallsyms with embedded PID
to prevent path traversal attacks.

Fixes: ebf0b332732d ("perf dso: fix dso__is_kallsyms() check")
Signed-off-by: Tanushree Shah <tshah@linux.ibm.com>
---
v2 -> v3: Replace strncmp() prefix matching with strcmp() for fixed
          kallsyms names and add is_guest_kallsyms_pid_name() to
          strictly validate guest kallsyms with PID format, preventing
          path traversal attacks.

v1 -> v2: Rename DSO__NAME_GUEST_KALLSYMS to DSO__PREFIX_GUEST_KALLSYMS
          to reflect that it is a prefix, not a full name.

v1: https://lore.kernel.org/all/20260410071225.708005-2-tshah@linux.ibm.com/

 tools/perf/util/dso.h | 57 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 56 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
index ede691e9a249..8763e6f65316 100644
--- a/tools/perf/util/dso.h
+++ b/tools/perf/util/dso.h
@@ -9,6 +9,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <linux/bitops.h>
+#include <string.h>
 #include "build-id.h"
 #include "debuginfo.h"
 #include "mutex.h"
@@ -20,6 +21,40 @@ struct perf_env;
 
 #define DSO__NAME_KALLSYMS	"[kernel.kallsyms]"
 #define DSO__NAME_KCORE		"[kernel.kcore]"
+#define DSO__NAME_GUEST_KALLSYMS		"[guest.kernel.kallsyms]"
+#define DSO__NAME_GUEST_KALLSYMS_PID_PREFIX	"[guest.kernel.kallsyms."
+
+/*
+ * Validate names of the form "[guest.kernel.kallsyms.<pid>]", where
+ * <pid> is the PID of the guest VM and varies per guest, so it
+ * cannot be matched with strcmp() against a fixed string.
+ *
+ * Every character after the fixed prefix must be a decimal digit,
+ * with ']' immediately terminating the digit run and nothing
+ * following it. This rules out '/', "..", or any other character
+ * being smuggled into the name.
+ */
+static inline bool is_guest_kallsyms_pid_name(const char *name)
+{
+	const size_t prefix_len = sizeof(DSO__NAME_GUEST_KALLSYMS_PID_PREFIX) - 1;
+	size_t digits;
+
+	if (strncmp(name, DSO__NAME_GUEST_KALLSYMS_PID_PREFIX, prefix_len) != 0)
+		return false;
+
+	digits = strspn(name + prefix_len, "0123456789");
+	if (digits == 0)
+		return false;
+
+	/* ']' must terminate the digit run, with nothing trailing it */
+	if (name[prefix_len + digits] != ']')
+		return false;
+
+	if (name[prefix_len + digits + 1] != '\0')
+		return false;
+
+	return true;
+}
 
 /**
  * enum dso_binary_type - The kind of DSO generally associated with a memory
@@ -914,8 +949,28 @@ static inline bool dso__is_kcore(const struct dso *dso)
 static inline bool dso__is_kallsyms(const struct dso *dso)
 {
 	enum dso_binary_type bt = dso__binary_type(dso);
+	const char *name;
+
+	if (bt == DSO_BINARY_TYPE__KALLSYMS || bt == DSO_BINARY_TYPE__GUEST_KALLSYMS)
+		return true;
+
+	if (bt != DSO_BINARY_TYPE__NOT_FOUND)
+		return false;
+
+	if (!RC_CHK_ACCESS(dso)->kernel)
+		return false;
+
+	name = RC_CHK_ACCESS(dso)->long_name;
+	if (!name)
+		return false;
+
+	if (!strcmp(name, DSO__NAME_KALLSYMS))
+		return true;
+
+	if (!strcmp(name, DSO__NAME_GUEST_KALLSYMS))
+		return true;
 
-	return bt == DSO_BINARY_TYPE__KALLSYMS || bt == DSO_BINARY_TYPE__GUEST_KALLSYMS;
+	return is_guest_kallsyms_pid_name(name);
 }
 
 bool dso__is_object_file(const struct dso *dso);
-- 
2.47.3

Re: [PATCH v3] perf dso: Fix kallsyms DSO detection with fallback logic

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Use-After-Free vulnerability due to lockless access of `dso->long_name` in `dso__is_kallsyms`.
--

commit 9c2571a16a38ac5caaaeb737772bc4cd229c357c
Author: Tanushree Shah <tshah@linux.ibm.com>

perf dso: Fix kallsyms DSO detection with fallback logic

This commit fixes an issue where kallsyms build-id symlinks fail to be
created during perf record/inject because the dso_binary_type is not
yet explicitly set. It falls back to matching the long_name against
known kallsyms strings when the binary type is not found. A strict
validation function is also added for guest kallsyms to prevent path
traversal attacks.

> diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
> index 2916b954a804b..884cca37753c4 100644
> --- a/tools/perf/util/dso.h
> +++ b/tools/perf/util/dso.h
[ ... ]
> @@ -924,8 +959,28 @@ static inline bool dso__is_kcore(const struct dso *dso)
>  static inline bool dso__is_kallsyms(const struct dso *dso)
>  {
>  	enum dso_binary_type bt = dso__binary_type(dso);
> +	const char *name;
> +
> +	if (bt == DSO_BINARY_TYPE__KALLSYMS || bt == DSO_BINARY_TYPE__GUEST_KALLSYMS)
> +		return true;
> +
> +	if (bt != DSO_BINARY_TYPE__NOT_FOUND)
> +		return false;
> +
> +	if (!RC_CHK_ACCESS(dso)->kernel)
> +		return false;
> +
> +	name = RC_CHK_ACCESS(dso)->long_name;
> +	if (!name)
> +		return false;
> +
> +	if (!strcmp(name, DSO__NAME_KALLSYMS))
> +		return true;

[Severity: High]
This isn't a bug introduced by this patch, but lockless access to 
dso->long_name is a systemic flaw that could lead to a use-after-free here.

If a concurrent thread modifies the DSO (e.g., the event processing thread in
perf top handling a ksymbol register event calls
machine__process_ksymbol_register() -> dso__set_long_name()), the old
long_name can be freed while the UI thread is executing dso__is_kallsyms()
and performing this strcmp().

Is it safe to expand the surface area of this pre-existing lockless string
dereference into a function that was previously safe?

> +
> +	if (!strcmp(name, DSO__NAME_GUEST_KALLSYMS))
> +		return true;
>  
> -	return bt == DSO_BINARY_TYPE__KALLSYMS || bt == DSO_BINARY_TYPE__GUEST_KALLSYMS;
> +	return is_guest_kallsyms_pid_name(name);
>  }
>  
>  bool dso__is_object_file(const struct dso *dso);

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260626161052.1024439-2-tshah@linux.ibm.com?part=1