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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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260505133922.797635-1-pratyush@kernel.org> X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: E832B100005 X-Rspam-User: X-Stat-Signature: ok58ybbmyyjsmt5gkpt91cmskm7mi9yw X-HE-Tag: 1777994877-19458 X-HE-Meta: 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 RLb/GUdg ExvR0K9Z5R+OCdsU8N33363YoowmxsxQd2e1weRiEb1dxZmLaCYs8uRG7ECX2x3kkRd4qqdHGrfkLwboyOSRMdh/aUH1yX5b+GP5Y0HhpvIso4Q4uc15aIgGMZ7IzXsdf34cHeTJFcxE1B/cMzMbVddyZUlRwM4Q8KarxTIdtkPPlB89V3XXmBoz7MyLbsM2s+48saiJ8kPqNTAijPAjh0O7mR1E3FnhI5GyQbSE2airuynKBnSETEFRon/U7/UE3pcGuT/87KlC+J0+v3yBRHjBsmMqcpleI26n0PkQ4F5r+NCKseUCyXZNhgSnlRmydKYPW4DnlwPglLXgRnrbR/aHSAEY3JK0u0ty6gb81Q4sHz6WE58vY7XWotz/by3eOCjbi9drSRvqc8Ymz3Yz/gfR+AIzldio7Kd58gSEosZUn2v08RTOBWaGGcU6rJyTgx0sw Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 >