linux-kernel - [PATCH RFC 3/4] fs/binfmt_elf: Tell user-space about the DENY_WRITE

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Date:   Wed, 13 Apr 2022 14:49:45 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Lennart Poettering <lennart@...ttering.net>,
        Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>
Cc:     Will Deacon <will@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        Mark Brown <broonie@...nel.org>,
        Jeremy Linton <jeremy.linton@....com>,
        Topi Miettinen <toiwoton@...il.com>, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-abi-devel@...ts.sourceforge.net
Subject: [PATCH RFC 3/4] fs/binfmt_elf: Tell user-space about the DENY_WRITE_EXEC personality flag

Since personality() accepts any flags and does not mask out any unknown
bits, inform user space that such personality flag is supported via an
AT_FLAGS_DENY_WRITE_EXEC flag.

Signed-off-by: Catalin Marinas <catalin.marinas@....com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Eric Biederman <ebiederm@...ssion.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Christoph Hellwig <hch@...radead.org>
---
 fs/binfmt_elf.c              | 2 ++
 include/uapi/linux/binfmts.h | 4 ++++
 2 files changed, 6 insertions(+)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 6556e13ed95f..4e6cba1f67ee 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -265,6 +265,8 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
 	NEW_AUX_ENT(AT_BASE, interp_load_addr);
 	if (bprm->interp_flags & BINPRM_FLAGS_PRESERVE_ARGV0)
 		flags |= AT_FLAGS_PRESERVE_ARGV0;
+	if (IS_ENABLED(CONFIG_ARCH_ENABLE_DENY_WRITE_EXEC))
+		flags |= AT_FLAGS_DENY_WRITE_EXEC;
 	NEW_AUX_ENT(AT_FLAGS, flags);
 	NEW_AUX_ENT(AT_ENTRY, e_entry);
 	NEW_AUX_ENT(AT_UID, from_kuid_munged(cred->user_ns, cred->uid));
diff --git a/include/uapi/linux/binfmts.h b/include/uapi/linux/binfmts.h
index c6f9450efc12..304bbb30264c 100644
--- a/include/uapi/linux/binfmts.h
+++ b/include/uapi/linux/binfmts.h
@@ -22,4 +22,8 @@ struct pt_regs;
 #define AT_FLAGS_PRESERVE_ARGV0_BIT 0
 #define AT_FLAGS_PRESERVE_ARGV0 (1 << AT_FLAGS_PRESERVE_ARGV0_BIT)
 
+/* support for DENY_WRITE_EXEC personality flag */
+#define AT_FLAGS_DENY_WRITE_EXEC_BIT 1
+#define AT_FLAGS_DENY_WRITE_EXEC (1 << AT_FLAGS_DENY_WRITE_EXEC_BIT)
+
 #endif /* _UAPI_LINUX_BINFMTS_H */