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Tue, 05 May 2026 08:27:56 -0700 (PDT) Date: Tue, 5 May 2026 15:27:55 +0000 From: Pasha Tatashin To: Pratyush Yadav Cc: Hugh Dickins , Baolin Wang , Andrew Morton , Jeff Xu , Kees Cook , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Pasha Tatashin , Brendan Jackman , Greg Thelen , stable@vger.kernel.org Subject: Re: [PATCH] memfd: deny writeable mappings when implying SEAL_WRITE Message-ID: References: <20260505133922.797635-1-pratyush@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260505133922.797635-1-pratyush@kernel.org> On 05-05 15:39, Pratyush Yadav wrote: > From: "Pratyush Yadav (Google)" > > When SEAL_EXEC is added, SEAL_WRITE is implied to make W^X. But the > implied seal is set after the check that makes sure the memfd can not > have any writable mappings. This means one can use SEAL_EXEC to apply > SEAL_WRITE while having writeable mappings. > > This breaks the contract that SEAL_WRITE provides and can be used by an > attacker to pass a memfd that appears to be write sealed but can still > be modified arbitrarily. > > Fix this by adding the implied seals before the call for > mapping_deny_writable() is done. > > Fixes: c4f75bc8bd6b ("mm/memfd: add write seals when apply SEAL_EXEC to executable memfd") > Cc: stable@vger.kernel.org > Signed-off-by: Pratyush Yadav (Google) Reviewed-by: Pasha Tatashin > --- > mm/memfd.c | 12 ++++++------ > 1 file changed, 6 insertions(+), 6 deletions(-) > > diff --git a/mm/memfd.c b/mm/memfd.c > index fb425f4e315f..abe13b291ddc 100644 > --- a/mm/memfd.c > +++ b/mm/memfd.c > @@ -283,6 +283,12 @@ int memfd_add_seals(struct file *file, unsigned int seals) > goto unlock; > } > > + /* > + * SEAL_EXEC implies SEAL_WRITE, making W^X from the start. > + */ > + if (seals & F_SEAL_EXEC && inode->i_mode & 0111) > + seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE; > + > if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) { > error = mapping_deny_writable(file->f_mapping); > if (error) > @@ -295,12 +301,6 @@ int memfd_add_seals(struct file *file, unsigned int seals) > } > } > > - /* > - * SEAL_EXEC implies SEAL_WRITE, making W^X from the start. > - */ > - if (seals & F_SEAL_EXEC && inode->i_mode & 0111) > - seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE; > - > *file_seals |= seals; > error = 0; > > -- > 2.54.0.545.g6539524ca2-goog >