libbpf: Fix the case of running as non-root with capabilities

When running rootless with special capabilities like:
FOWNER / DAC_OVERRIDE / DAC_READ_SEARCH

The "access" API will not make the proper check if there is really
access to a file or not.

>From the access man page:
"
The check is done using the calling process's real UID and GID, rather
than the effective IDs as is done when actually attempting an operation
(e.g., open(2)) on the file.  Similarly, for the root user, the check
uses the set of permitted capabilities  rather than the set of effective
capabilities; ***and for non-root users, the check uses an empty set of
capabilities.***
"

What that means is that for non-root user the access API will not do the
proper validation if the process really has permission to a file or not.

To resolve this this patch replaces all the access API calls with
faccessat with AT_EACCESS flag.

Signed-off-by: Jon Doron <jond@wiz.io>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220925070431.1313680-1-arilou@gmail.com

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index b4d9a96c3c1bf75b6c1747a22f304425655a7c36..d88647da2c7fc599780a250cae309632ba9e2fb8 100644 (file)
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -4664,7 +4664,7 @@ struct btf *btf__load_vmlinux_btf(void)
        for (i = 0; i < ARRAY_SIZE(locations); i++) {
                snprintf(path, PATH_MAX, locations[i], buf.release);
 
-               if (access(path, R_OK))
+               if (faccessat(AT_FDCWD, path, R_OK, AT_EACCESS))
                        continue;
 
                btf = btf__parse(path, NULL);

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index e691f08a297ff01243a54f8cfb09d3ef0eaf0004..184ce1684dcd4c46b0cf4525f0b80047c5446692 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -884,7 +884,7 @@ __u32 get_kernel_version(void)
        __u32 major, minor, patch;
        struct utsname info;
 
-       if (access(ubuntu_kver_file, R_OK) == 0) {
+       if (faccessat(AT_FDCWD, ubuntu_kver_file, R_OK, AT_EACCESS) == 0) {
                FILE *f;
 
                f = fopen(ubuntu_kver_file, "r");
@@ -9904,7 +9904,7 @@ static bool use_debugfs(void)
        static int has_debugfs = -1;
 
        if (has_debugfs < 0)
-               has_debugfs = access(DEBUGFS, F_OK) == 0;
+               has_debugfs = faccessat(AT_FDCWD, DEBUGFS, F_OK, AT_EACCESS) == 0;
 
        return has_debugfs == 1;
 }
@@ -10721,7 +10721,7 @@ static int resolve_full_path(const char *file, char *result, size_t result_sz)
                                continue;
                        snprintf(result, result_sz, "%.*s/%s", seg_len, s, file);
                        /* ensure it has required permissions */
-                       if (access(result, perm) < 0)
+                       if (faccessat(AT_FDCWD, result, perm, AT_EACCESS) < 0)
                                continue;
                        pr_debug("resolved '%s' to '%s'\n", file, result);
                        return 0;

diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
index d18e37982344c7c85b4a31d896f580d14927c892..e83b497c2245443473539a5a25c4ca98f13257fc 100644 (file)
--- a/tools/lib/bpf/usdt.c
+++ b/tools/lib/bpf/usdt.c
@@ -282,7 +282,7 @@ struct usdt_manager *usdt_manager_new(struct bpf_object *obj)
         * If this is not supported, USDTs with semaphores will not be supported.
         * Added in: a6ca88b241d5 ("trace_uprobe: support reference counter in fd-based uprobe")
         */
-       man->has_sema_refcnt = access(ref_ctr_sysfs_path, F_OK) == 0;
+       man->has_sema_refcnt = faccessat(AT_FDCWD, ref_ctr_sysfs_path, F_OK, AT_EACCESS) == 0;
 
        return man;
 }